Method of manufacturing semiconductor device

ABSTRACT

Provided are an oxide semiconductor layer in which the number of defects is reduced and a highly reliable semiconductor device including the oxide semiconductor. A first oxide semiconductor layer containing a single metal element as a constituent element is formed over a substrate by a thermal chemical vapor deposition method. A second oxide semiconductor layer containing two or more metal elements as constituent elements is formed successively after the first oxide semiconductor layer is formed. The second oxide semiconductor layer is formed by epitaxial growth using the first oxide semiconductor layer as a seed crystal. A channel is formed in the second oxide semiconductor layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a semiconductor device or a methodof manufacturing the semiconductor device.

In this specification and the like, the term “semiconductor device”means all devices that can operate by utilizing semiconductorcharacteristics. An electro-optical device, an image display device(also referred to as a display device), a semiconductor circuit, alight-emitting device, a power storage device, a memory device, or anelectronic device include a semiconductor device in some cases.

2. Description of the Related Art

A technique in which a transistor is formed using a semiconductor layerhas attracted attention. The transistor is used in a wide range ofelectronic devices such as an integrated circuit (IC) and a displaydevice. As semiconductor materials applicable to the transistor,silicon-based semiconductor materials have been widely used, but oxidesemiconductors have attracted attention as alternative materials.

Formation of an oxide semiconductor film having crystallinity has beenstudied. For example, according to Non-Patent Document 1, anInGaO₃(ZnO)₅ thin film is formed by a PLD method over a single crystalthin film of ZnO formed over a YSZ substrate, and subjected to heattreatment at 1400° C. to have a single crystal structure. According to atechnique disclosed in Patent Document 1, a single-component oxidesemiconductor layer is formed over a substrate; crystal growth iscarried out from a surface to an inside by heat treatment at 500° C. orhigher and 1000° C. or lower, so that the single-component oxidesemiconductor layer includes a single crystal region; and amulti-component oxide semiconductor layer including a single crystalregion is formed over the single-component oxide semiconductor layerincluding a single crystal region. The multi-component oxidesemiconductor layer formed in this manner is used for a transistor.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2011-146698

Non-Patent Document

-   [Non-Patent Document 1] Kenji Nomura et al., J. Appl. Phys. Vol. 95,    pp. 5532-5539 (2004)

SUMMARY OF THE INVENTION

In a transistor including an oxide semiconductor, a defect in an oxidesemiconductor layer or a bond of the defect with hydrogen or the likemight cause a carrier in a film, which might change the electricalcharacteristics of the transistor.

In view of the above, an object of one embodiment of the presentinvention is to provide an oxide semiconductor layer in which the numberof defects is reduced. Another object is to improve the reliability of atransistor including an oxide semiconductor. Another object is toimprove electrical characteristics of a semiconductor device or the likeincluding an oxide semiconductor. Another object is to provide a novelsemiconductor device or the like.

Note that the descriptions of these objects do not disturb the existenceof other objects. Note that in one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

An oxygen vacancy is given as a carrier supply source of an oxidesemiconductor. Oxygen vacancies contained in the oxide semiconductorappear as localized states in deep energy levels in the energy gap ofthe oxide semiconductor. When carriers are trapped by such localizedstates, electrical characteristics of the transistor degrade; forexample, the transistor becomes normally-on, or has an increased leakagecurrent or a threshold voltage shifted by stress application. To improvethe reliability of the transistor, it is necessary to reduce the numberof oxygen vacancies in the oxide semiconductor.

A factor that forms an oxygen vacancy in an oxide semiconductor layer isplasma damage during formation of the oxide semiconductor layer. Forexample, in the case where plasma is used to form an oxide semiconductorlayer containing indium, the oxide semiconductor layer might be damagedby the plasma and an In—O—In bond having the weakest bonding might becut, which causes an oxygen vacancy.

Accordingly, in one embodiment of the present invention, a methodwithout using plasma is employed for forming an oxide semiconductorlayer in which a channel is formed; thus, plasma damage to the oxidesemiconductor layer is prevented and the oxide semiconductor layer hasfewer defects. The method without using plasma is, for example, athermal chemical vapor deposition (thermal CVD) method. In oneembodiment of the present invention, by epitaxial growth in which thefirst oxide semiconductor layer containing a single metal element as aconstituent element is used as a seed crystal, the second oxidesemiconductor layer containing two or more metal elements as constituentelements is formed. The first oxide semiconductor layer containing asingle metal element as a constituent element is crystallized moreeasily than the second oxide semiconductor layer containing two or moremetal elements as constituent elements, and serves a seed crystal havinghigh crystallinity. Thus, the second oxide semiconductor layer in whichplasma damage is suppressed and which has high crystallinity can beformed with high productivity.

More specifically, for example, the following manufacturing method canbe employed.

One embodiment of the present invention is a method of manufacturing asemiconductor device, including the steps of: forming a first oxidesemiconductor layer containing a single metal element as a constituentelement over a substrate by a thermal chemical vapor deposition method;and forming a second oxide semiconductor layer containing two or moremetal elements as constituent elements successively after the firstoxide semiconductor layer is formed. The second oxide semiconductorlayer is formed by epitaxial growth using the first oxide semiconductorlayer as a seed crystal.

Another embodiment of the present invention is a method of manufacturinga semiconductor device, including the steps of: forming a first oxidesemiconductor layer containing a single metal element as a constituentelement over a substrate by a metal organic chemical vapor depositionmethod; and forming a second oxide semiconductor layer containing two ormore metal elements as constituent elements successively after the firstoxide semiconductor layer is formed. The second oxide semiconductorlayer is formed by epitaxial growth using the first oxide semiconductorlayer as a seed crystal.

In the methods of manufacturing a semiconductor device, the first oxidesemiconductor layer and the second oxide semiconductor layer arepreferably formed in the same deposition chamber.

Another embodiment of the present invention is a method of manufacturinga semiconductor device, including the steps of: forming a first oxidesemiconductor layer containing a single metal element as a constituentelement over a substrate by a thermal chemical vapor deposition method;forming a second oxide semiconductor layer containing two or more metalelements as constituent elements successively after the first oxidesemiconductor layer is formed; forming a source electrode layer and adrain electrode layer which are electrically connected to the secondoxide semiconductor layer; forming a gate insulating layer in contactwith the second oxide semiconductor layer; and forming a gate electrodelayer facing the second oxide semiconductor layer with the gateinsulating layer located therebetween. The second oxide semiconductorlayer is formed by epitaxial growth using the first oxide semiconductorlayer as a seed crystal.

Another embodiment of the present invention is a method of manufacturinga semiconductor device, including the steps of: forming a gate electrodelayer over a substrate; forming a gate insulating layer in contact withthe gate electrode layer; forming a first oxide semiconductor layercontaining a single metal element as a constituent element in a positionfacing the gate electrode layer with the gate insulating layer locatedtherebetween, by a thermal chemical vapor deposition method; forming asecond oxide semiconductor layer containing two or more metal elementsas constituent elements successively after the first oxide semiconductorlayer is formed; and forming a source electrode layer and a drainelectrode layer which are electrically connected to the second oxidesemiconductor layer. The second oxide semiconductor layer is formed byepitaxial growth using the first oxide semiconductor layer as a seedcrystal.

In the methods of manufacturing a semiconductor device, a metal organicchemical vapor deposition method can be used as the thermal chemicalvapor deposition method.

In the methods of manufacturing a semiconductor device, the thickness ofthe first oxide semiconductor layer can be 0.1-atomic-layer thick orlarger and 20-atomic-layer thick or smaller.

In the methods of manufacturing a semiconductor device, a film having aregion whose electron affinity is higher than an electron affinity ofthe first oxide semiconductor layer is preferably formed as the secondoxide semiconductor layer.

In the methods of manufacturing a semiconductor device, a zinc oxidelayer having a hexagonal-crystal structure at the surface is preferablyformed as the first oxide semiconductor layer.

In the method of manufacturing a semiconductor device, an In-M-Zn oxidelayer (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) is preferably formedas the second oxide semiconductor layer.

In the method of manufacturing a semiconductor device, the temperaturefor forming the first oxide semiconductor layer and the second oxidesemiconductor layer may be 400° C. or higher and 800° C. or lower.

One embodiment of the present invention can provide an oxidesemiconductor layer in which the number of defects is reduced. Oneembodiment of the present invention can improve the reliability of atransistor including an oxide semiconductor. One embodiment of thepresent invention can improve electrical characteristics of asemiconductor device or the like including an oxide semiconductor. Notethat one embodiment of the present invention is not limited to theseeffects. For example, depending on circumstances or conditions, oneembodiment of the present invention might produce another effect.Furthermore, depending on circumstances or conditions, one embodiment ofthe present invention might not produce the above effects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D illustrate a method of forming an oxide semiconductorlayer of one embodiment of the present invention.

FIGS. 2A and 2B are schematic views illustrating an oxide semiconductorlayer of one embodiment of the present invention.

FIGS. 3A to 3C each illustrate a band structure of oxide semiconductorlayers of one embodiment of the present invention.

FIGS. 4A and 4B illustrate an apparatus for manufacturing asemiconductor device of one embodiment of the present invention.

FIGS. 5A and 5B are a plan view and a cross-sectional view illustratinga transistor of one embodiment of the present invention.

FIGS. 6A and 6B are a plan view and a cross-sectional view illustratinga transistor of one embodiment of the present invention.

FIGS. 7A and 7B are a plan view and a cross-sectional view illustratinga transistor of one embodiment of the present invention.

FIGS. 8A and 8B are a plan view and a cross-sectional view illustratinga transistor of one embodiment of the present invention.

FIGS. 9A and 9D are cross-sectional views and FIGS. 9B and 9C arecircuit diagrams of a semiconductor device of one embodiment of thepresent invention.

FIGS. 10A and 10B are each a circuit diagram of a memory device of oneembodiment of the present invention.

FIG. 11 is a block diagram of an RF tag of one embodiment of the presentinvention.

FIGS. 12A to 12F illustrate application examples of an RF tag of oneembodiment of the present invention.

FIG. 13 is a block diagram illustrating a CPU of one embodiment of thepresent invention.

FIG. 14 is a circuit diagram of a memory element of one embodiment ofthe present invention.

FIG. 15A illustrates a structure of a display device of one embodimentof the present invention and FIGS. 15B and 15C are circuit diagramsthereof.

FIGS. 16A to 16F illustrate electronic devices of one embodiment of thepresent invention.

FIGS. 17A and 17B are cross-sectional views illustrating a transistor ofone embodiment of the present invention.

FIGS. 18A and 18B are cross-sectional views illustrating a transistor ofone embodiment of the present invention.

FIGS. 19A and 19B are cross-sectional views illustrating a transistor ofone embodiment of the present invention.

FIGS. 20A to 20D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS layer and a cross-sectional schematic view of theCAAC-OS layer.

FIGS. 21A to 21D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS layer.

FIGS. 22A to 22C show structural analysis of a CAAC-OS layer and asingle crystal oxide semiconductor by XRD.

FIGS. 23A and 23B show electron diffraction patterns of a CAAC-OS layer.

FIG. 24 shows a change of crystal parts of an In—Ga—Zn oxide owing toelectron irradiation.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description. It will be readily understood bythose skilled in the art that modes and details of the present inventioncan be changed in various ways without departing from the spirit andscope of the present invention. Therefore, the present invention shouldnot be interpreted as being limited to the description of the followingembodiments. In addition, in the following embodiments, the sameportions or portions having similar functions are denoted by the samereference numerals or the same hatching patterns in different drawings,and description thereof will not be repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the scale is not necessarily limitedto that illustrated in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and the like are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.Therefore, for example, description can be made even when “first” isreplaced with “second” or “third”, as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with eachother when the direction of current flowing is changed in circuitoperation, for example. Therefore, the terms “source” and “drain” can bereplaced with each other in this specification and the like.

In this specification and the like, the terms “over” and “below” do notnecessarily mean that a component is placed “directly on” or “directlyunder” another component. For example, the expression “a gate electrodelayer over a gate insulating film” can mean the case where there is anadditional component between the gate insulating film and the gateelectrode layer. The same applies to the term “below”.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly includes the casewhere the angle is greater than or equal to −5° and less than or equalto 5°. The term “substantially parallel” indicates that an angle formedbetween two straight lines is greater than or equal to −30 and less thanor equal to 30°. The term “perpendicular” indicates that the angleformed between two straight lines is greater than or equal to 80° andless than or equal to 100°, and accordingly includes the case where theangle is greater than or equal to 85° and less than or equal to 95°. Theterm “substantially perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 60° and less thanor equal to 120°.

In this specification and the like, the trigonal and rhombohedralcrystal systems are included in the hexagonal crystal system.

(Embodiment 1)

In this embodiment, a method of forming an oxide semiconductor layerthat can be used in a semiconductor device of one embodiment of thepresent invention will be described with reference to drawings.

In one embodiment of the present invention, a first oxide semiconductorlayer containing a single metal element as a constituent element isformed over a substrate, and a second oxide semiconductor layercontaining two or more metal elements as constituent elements is formedby epitaxial growth using the first oxide semiconductor layer as a seedcrystal. Here, a thermal CVD method is employed for the formation of thefirst oxide semiconductor layer and the second oxide semiconductorlayer. The thermal CVD method is different from a plasma CVD method bywhich a source gas of a main component of a film is decomposed byplasma. By the thermal CVD method, a source gas is decomposed by heatingover a heated substrate; thus, in the oxide semiconductor layer formedby the thermal CVD method, plasma damage is prevented and the number ofdefects is reduced. Furthermore, plasma damage at an interface between alower layer of the oxide semiconductor layer and an insulating layer incontact with the lower layer can be inhibited. For example, in the casewhere a channel of a transistor is formed in the oxide semiconductorlayer, change in electrical characteristics of the transistor can besuppressed.

Note that the thermal CVD method can be classified into a metal CVD(MCVD) method and a metal organic CVD (MOCVD) method depending on asource gas. To form oxide semiconductor layers having a crystal part asthe first oxide semiconductor layer and the second oxide semiconductorlayer, it is preferable to use an MOCVD method by which a gas containingan organic metal that is a main component is decomposed and deposited.Here, a thermal decomposition method is employed for decomposing a gascontaining an organic metal. The methods of forming the first oxidesemiconductor layer and the second oxide semiconductor layer are notlimited to these methods, and a method by which plasma damage during thedeposition is prevented or reduced can be used as appropriate. Forexample, an atomic layer deposition (ALD) method may be used.

<Method of Forming Stacked Structure Including Oxide SemiconductorLayers>

A method of forming an oxide semiconductor layer of one embodiment ofthe present invention and an insulating layer in contact with the oxidesemiconductor layer will be described below with reference to FIGS. 1Ato 1D and FIGS. 2A and 2B.

First, a substrate 100 is prepared. The substrate 100 is not limited toa single crystal substrate as long as the substrate has heat resistancehigh enough to withstand heat treatment in the process of manufacturinga semiconductor device. For example, a glass substrate, a ceramicsubstrate, a quartz substrate, a sapphire substrate, anyttria-stabilized zirconia (YSZ) substrate, an SOI substrate, or asubstrate of a semiconductor such as silicon, silicon carbide, galliumnitride, or gallium oxide can be used as the substrate 100, asappropriate.

A first insulating layer 102 is formed over the substrate 100 (see FIG.1A). The first insulating layer 102 has a function of preventingimpurity diffusion from the substrate 100. The first insulating layer102 preferably contains oxygen, more preferably contains oxygen morethan that in the stoichiometric composition, in which case, oxygen canbe supplied to a first oxide semiconductor layer or a second oxidesemiconductor layer to be formed later over the first insulating layer102.

As the first insulating layer 102, for example, a single layer or astacked layer of an insulating film containing aluminum oxide, magnesiumoxide, silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide maybe used.

A surface of the first insulating layer 102 may be planarized. Forexample, the first insulating layer 102 may be subjected toplanarization treatment such as chemical mechanical polishing (CMP)treatment. By the CMP treatment, the first insulating layer 102 has anaverage surface roughness (R_(a)) of less than or equal to 1 nm,preferably less than or equal to 0.3 nm, more preferably less than orequal to 0.1 nm. With R_(a) of less than or equal to the above value,the crystallinity of a first oxide semiconductor layer 104 and/or asecond oxide semiconductor layer 106 to be formed later may be improved.R_(a) can be measured with an atomic force microscope (AFM). Note thatthe first insulating layer 102 is not necessarily formed.

The first insulating layer 102 can be formed by a sputtering method, aCVD method, a molecular beam epitaxy (MBE) method, an atomic layerdeposition (ALD) method, or a pulsed laser deposition (PLD) method, forexample. A thermal CVD method is preferably used to reduce plasma damageto the deposited film or the first or second oxide semiconductor layerformed over the first insulating layer 102.

Alternatively, in the case where a silicon substrate is used as thesubstrate 100, the insulating layer to be the first insulating layer 102may be formed by a thermal oxidation method.

Next, the first oxide semiconductor layer 104 is formed over the firstinsulating layer 102 (see FIG. 1B).

An oxide semiconductor material containing a single metal element as aconstituent element is preferably used for the first oxide semiconductorlayer 104. More preferably, an oxide semiconductor material whichcontains a single metal element as a constituent element and which canhave a hexagonal-crystal structure is used. The first oxidesemiconductor layer 104 needs to have a crystal part, preferably asingle crystal region because it is used as a seed crystal for formingthe second oxide semiconductor layer to be formed later. The oxidesemiconductor material containing a single metal element as aconstituent element is crystallized more easily and can have highercrystallinity than an oxide semiconductor material containing two ormore metal elements as constituent elements does. For this reason, theoxide semiconductor material containing a single metal element can besuitably used for the first oxide semiconductor layer 104. A typicalexample of the oxide semiconductor material is a zinc oxide.

Note that the oxide semiconductor material containing a single metalelement as a constituent element (main component) may contain an elementother than the metal element and oxygen as impurity. In that case, theconcentration of the impurity is preferably 0.1% or lower. For example,when a zinc oxide layer is formed as the first oxide semiconductor layer104, it might contain an element other than zinc and oxygen as impurityat 0.1% or lower. The same applies to an oxide semiconductor materialcontaining two or more metal elements as constituent elements.

Alternatively, the first oxide semiconductor layer 104 is preferablyformed with an oxide semiconductor material whose lattice constant in asingle crystal state is close to that of the second oxide semiconductorlayer in a single crystal state. The first oxide semiconductor layer 104has a lattice constant close to that of the second oxide semiconductorlayer, so that lattice mismatch with the second oxide semiconductorlayer can be small and generation of lattice defects can be suppressed.

The case where a zinc oxide layer is formed as the first oxidesemiconductor layer 104 is described below in this embodiment. Note thatthe first oxide semiconductor layer 104 is not limited to a zinc oxidelayer.

The first oxide semiconductor layer 104 can be formed by a methodwithout using plasma. In this embodiment, the zinc oxide layer is formedby an MOCVD method using an organometallic gas containing zinc and a gascontaining oxygen. As the organometallic gas containing zinc,diethylzinc or dimethylzinc can be used, for example. As the gascontaining oxygen, a dinitrogen monoxide gas, an oxygen gas, or ozonecan be used, for example. Note that source gases that can be used arenot limited thereto.

The first oxide semiconductor layer 104 is used as a seed crystal forcrystal growth of the second oxide semiconductor layer to be formedlater. Therefore, the thickness of the first oxide semiconductor layer104 may be a thickness with which crystal growth is possible, andtypically 0.1-atomic-layer thick or larger and 20-atomic-layer thick orsmaller, preferably 1-atomic-layer thick or larger and 5-atomic-layerthick or smaller. The first oxide semiconductor layer 104 is preferablythin, in which case throughput in deposition can be improved. Note thatin this specification and the like, the expression “the thickness of thefirst oxide semiconductor layer 104 is 0.1-atomic-layer thick or largerand less than 1-atomic-layer thick” can mean that the layer whosethickness is 1-atomic-layer thick or larger is partly deposited in afilm formation region.

The atmosphere for forming the first oxide semiconductor layer 104 canbe an inert gas atmosphere such as a rare gas (typically argon). Toprevent impurity from entering the first oxide semiconductor layer 104,a deposition gas is preferably highly purified. It is preferable tohighly purify a gas containing oxygen and an inert gas used asdeposition gases such that the dew point is −40° C. or lower, preferably−80° C. or lower, more preferably −100° C. or lower, still morepreferably −120° C. or lower.

The substrate temperature for forming the first oxide semiconductorlayer 104 is 110° C. or higher, preferably 150° C. or higher, morepreferably 200° C. or higher. For example, when a zinc oxide layer isformed as the first oxide semiconductor layer 104 at a substratetemperature of 110° C. or higher, oxygen particles and zinc particlesmove over a surface on which a film is to be formed and form ahexagonal-crystal zinc oxide. Crystal growth of a zinc oxide is fast inthe direction parallel to the a-b plane. When the substrate temperatureis 110° C. or higher, preferably 150° C. or higher, more preferably 200°C. or higher, crystal growth of the hexagonal-crystal zinc oxideproceeds in the direction parallel to the surface of the substrate 100(i.e., the lateral direction in the cross section of the zinc oxidelayer). As a result, a hexagonal-crystal zinc oxide layer which has asingle crystal region is formed. By setting the substrate temperaturehigh, impurity in the first oxide semiconductor layer 104 can bereduced: thus, the crystallinity of the first oxide semiconductor layer104 can be improved. For example, when an oxide semiconductor layerwhich is single crystal or substantially single crystal is formed as thefirst oxide semiconductor layer 104, the substrate temperature is set to400° C. or higher, preferably 500° C. or higher.

In terms of deposition rate and productivity, the substrate temperatureis preferably 800° C. or lower. A zinc oxide is likely to evaporateunder a reduced-pressure atmosphere at 600° C. or higher. For thisreason, when the substrate temperature is 600° C. or higher, evaporationof a zinc oxide partly occurs, so that the hexagonal-crystal zinc oxidelayer might have a region where the zinc oxide layer is not formed (aregion where the first insulating layer 102 is exposed) and do not coverthe entire surface of the substrate 100. Thus, the substrate temperatureis preferably lower than 600° C.

FIG. 2A illustrates a model of a top-view shape of the hexagonal-crystalzinc oxide layer, and FIG. 2B illustrates a model of a cross-sectionalshape thereof. As illustrated in FIGS. 2A and 2B, Zn atoms and O atomsare bound in a hexagonal shape in the hexagonal-crystal zinc oxidelayer. The binding of Zn atoms and O atoms in the hexagonal shapeextends over the a-b plane. As described above, since crystal growth ofa zinc oxide is fast in the direction parallel to the a-b plane, crystalgrowth proceeds first in the direction parallel to the surface (a-bplane). At this time, in the a-b plane, crystal growth occurs in thelateral direction to form a single crystal region. Next, crystal growthproceeds in the c-axis direction which is perpendicular to the surface(a-b plane), using the single crystal region formed in the surface (a-bplane) as a seed crystal. Accordingly, a zinc oxide is likely to havec-axis alignment.

Next, by epitaxial growth using the first oxide semiconductor layer 104as a crystal seed, a second oxide semiconductor layer 106 is formed overthe first oxide semiconductor layer 104 (see FIG. 1C). The second oxidesemiconductor layer 106 is formed by a method without using plasma(e.g., a thermal CVD method).

The second oxide semiconductor layer 106 is formed successively afterthe first oxide semiconductor layer 104 is formed, without exposure tothe air. Thus, an impurity (e.g., hydrogen or a hydrogen compound inadsorbed water) can be prevented from entering an interface between thefirst oxide semiconductor layer 104 and the second oxide semiconductorlayer 106. Note that although the first oxide semiconductor layer 104and the second oxide semiconductor layer 106 may be formed in differentdeposition chambers as long as they are not exposed to the air, it ispreferable that the first oxide semiconductor layer 104 and the secondoxide semiconductor layer 106 be formed successively in the same chamberbecause the productivity of a semiconductor device can be improved. Inaddition, entry of impurity at the time of transfer can be prevented.

In the case where the first oxide semiconductor layer 104 and the secondoxide semiconductor layer 106 are formed in the same deposition chamber,it is preferable that deposition conditions (e.g., depositiontemperature) for the oxide semiconductor layers be partly or entirelythe same because the productivity is improved. Furthermore, it ispreferable that the first oxide semiconductor layer 104 and the secondoxide semiconductor layer 106 be both formed by an MOCVD method, inwhich case they can have different compositions by changing the flowrates of source gases.

For the second oxide semiconductor layer 106, an oxide semiconductormaterial containing two or more metal elements as constituent elementsis preferably used; an oxide semiconductor material containing at leastindium as a constituent element is more preferably used; an oxidesemiconductor material containing at least indium and zinc asconstituent elements is still more preferably used. Still furtherpreferably, an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf)is used for the second oxide semiconductor layer 106. The element M isan element that can increase the energy gap of an oxide, for example.Thus, the energy gap of the oxide can be controlled by adjusting thecomposition of M in the In-M-Zn oxide.

The atmosphere in the formation of the second oxide semiconductor layer106 can be an inert gas (e.g., a rare gas) atmosphere. As in theformation of the first oxide semiconductor layer 104, it is preferablethat a deposition chamber be highly vacuum-evacuated and a depositiongas be highly purified in order to prevent an impurity from entering thesecond oxide semiconductor layer 106.

In this embodiment, an In—Ga—Zn oxide layer is formed as the secondoxide semiconductor layer 106 by an MOCVD method using an organometallicgas containing indium, an organometallic gas containing gallium, anorganometallic gas containing zinc, and a gas containing oxygen. Forexample, trimethylindium can be used as the organometallic materialcontaining indium; trimethylgallium or triethylgallium can be used asthe organometallic material containing gallium; diethylzinc ordimethylzinc can be used as the organometallic material containing zinc;and a dinitrogen monoxide gas, an oxygen gas, or ozone can be used asthe gas containing oxygen. The hexagonal-crystal zinc oxide layer hashigh crystallinity and the lattice constant in the a-b plane directionis close to that of an In—Ga—Zn oxide. Since a hexagonal-crystal zincoxide has a hexagonal lattice structure, epitaxial growth of theIn—Ga—Zn oxide having a hexagonal lattice structure can be carried outover the zinc oxide layer. Therefore, with use of the hexagonal-crystalzinc oxide layer as a seed crystal, the In—Ga—Zn oxide layer having highcrystallinity can be firmed. Note that the source gases that can be usedare not limited to these. Furthermore, an oxide semiconductor materialother than the In—Ga—Zn oxide can be used for the second oxidesemiconductor layer 106.

Since the first oxide semiconductor layer 104 has a hexagonal-crystalstructure at its surface, when the mixed source gas described abovereaches the surface of the first oxide semiconductor layer 104 while thesecond oxide semiconductor layer 106 is deposited, thermal decompositionand/or chemical reaction occurs, and the second oxide semiconductorlayer 106 is formed in the same manner as the first oxide semiconductorlayer 104. Thus, the second oxide semiconductor layer 106 with highcrystallinity can be formed even with an oxide semiconductor materialwhich contains two or more metal elements as constituent elements andwhich is relatively difficult to crystallize. When the first oxidesemiconductor layer 104 has a single crystal region or is a singlecrystal layer (substantially a single crystal layer), the second oxidesemiconductor layer 106, which results from the epitaxial growth usingthe first oxide semiconductor layer 104, also can have a single crystalregion or can be a single crystal layer (substantially a single crystallayer).

The substrate temperature for forming the second oxide semiconductorlayer 106 is 110° C. or higher, preferably 150° C. or higher, morepreferably 200° C. or higher. By setting the substrate temperature high,impurities in the second oxide semiconductor layer 106 can be reduced;thus, the crystallinity of the second oxide semiconductor layer 106 canbe improved. For example, in the case where an oxide semiconductor layerwhich is a single crystal layer or substantially a single crystal layeris formed as the second oxide semiconductor layer 106, the substratetemperature is 400° C. or higher, preferably 500° C. or higher. In termsof deposition rate and productivity, the substrate temperature ispreferably 800° C. or lower. In terms of evaporation of a zinc oxide,the substrate temperature is preferably lower than 600° C.

Next, a second insulating layer 110 is formed over the second oxidesemiconductor layer 106 (see FIG. 1D).

As the second insulating layer 110, for example, a single layer or astacked layer of an insulating film containing aluminum oxide, magnesiumoxide, silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide maybe used.

The second insulating layer 110 may be formed by a sputtering method, aCVD method, a pulsed laser deposition method, or the like. It ispreferable to use a thermal CVD method to reduce plasma damage to thedeposited film or the first oxide semiconductor layer 104 or the secondoxide semiconductor layer 106 under the second insulating layer 110.

Through the above steps, a stacked structure including oxidesemiconductor layers of one embodiment of the present invention can beformed.

By a CVD method, the composition of a film can be controlled by a flowrate of a source gas. For example, by an MCVD method or an MOCVD method,a film with a desired composition can be formed by adjusting the flowrate of a source gas. Furthermore, for example, by an MCVD method or anMOCVD method, a film whose composition is gradually changed can beformed by changing the flow rate of a source gas during deposition. Inthe case where a film is deposited while the flow rate of a source gasis changed, the time for film formation can be shorter than in the casewhere a film is deposited using a plurality of deposition chambersbecause time for transferring the substrate and time for adjusting thepressure are not needed. Thus, a semiconductor device can bemanufactured with improved productivity.

For example, the second oxide semiconductor layer 106 is deposited whilethe flow rate of a source gas is changed, in which case the compositionof the second oxide semiconductor layer 106 can be changed in thethickness direction. In the case where an In-M-Zn oxide layer is formedas the second oxide semiconductor layer 106, the band structure of thesecond oxide semiconductor layer 106 can be controlled by adjusting theproportion of the element M which has a function of making the energygap of an oxide large. Specifically, for example, in the case where anIn—Ga—Zn oxide layer is formed as the second oxide semiconductor layer106, by setting the flow rate of an organometallic gas containing Ga tobe small, the oxide semiconductor layer has a small band gap. In thiscase, the electron affinity (an energy difference between the vacuumlevel and the bottom of the conduction band) of the second oxidesemiconductor layer 106 can be high. The energy at the top of thevalence band of the In—Ga—Zn oxide can be controlled by adjusting theproportion of zinc, for example. In the case where an In—Ga—Zn oxidelayer is formed as the second oxide semiconductor layer 106, by settingthe flow rate of an organometallic gas containing Ga to be large, theoxide semiconductor layer has a large band gap. In this case, theelectron affinity of the second oxide semiconductor layer 106 can below.

FIGS. 3A to 3C illustrate examples of a band structure of the secondoxide semiconductor layer 106 in the case where the flow rate of asource gas is changed during the deposition.

FIGS. 3A to 3C each illustrate a band structure of a stacked structurein the thickness direction including the first insulating layer 102, thefirst oxide semiconductor layer 104, the second oxide semiconductorlayer 106, and the second insulating layer 110. For easy understanding,each band structure shows energy (Ec) at bottoms of conduction bands ofthe first insulating layer 102, the first oxide semiconductor layer 104,the second oxide semiconductor layer 106, and the second insulatinglayer 110.

In the band structure of the second oxide semiconductor layer 106 inFIG. 3A, energy is continuously changed near an interface with the firstoxide semiconductor layer 104 (continuous junction). Specifically, nearthe interface with the first oxide semiconductor layer 104, the electronaffinity becomes larger as the bottom of the conduction band of thesecond oxide semiconductor layer 106 is farther from the first oxidesemiconductor layer 104. Near the interface with the second insulatinglayer 110, energy at the bottom of the conduction band of the secondoxide semiconductor layer 106 is constant, and the electron affinity isalso constant.

For example, in the case where an In-M-Zn oxide layer (M is Al, Ti, Ga,Y, Zr, La, Ce, Nd, or Hf) is formed as the second oxide semiconductorlayer 106, the second oxide semiconductor layer 106 has a concentrationgradient such that, near the interface with the first oxidesemiconductor layer 104, the concentration of the element M is lower asthe second oxide semiconductor layer 106 is farther from the first oxidesemiconductor layer 104, and near the interface with the secondinsulating layer 110, the concentration of the element M is constant (orsubstantially constant). In that case, the band structure of the secondoxide semiconductor layer 106 illustrated in FIG. 3A might be formed.Note that the concentration of the element M in the second oxidesemiconductor layer 106 can be measured by secondary ion massspectrometry (SIMS), for example.

In the case where an insulating layer including a constituent element(e.g., silicon) which is different from that of an oxide semiconductoris provided as the first insulating layer 102, an interface state due tojunction of different kinds of materials, entry of an impurity, or thelike might be formed at the interface with the first oxide semiconductorlayer 104. In the case where the first insulating layer 102 is formed bya method using plasma (e.g., a plasma CVD method or a sputteringmethod), an interface state due to plasma damage might be formed in thefirst insulating layer 102 or near the interface between the firstinsulating layer 102 and the first oxide semiconductor layer 104. Sincethe first oxide semiconductor layer 104 is used as a seed crystal andhas an extremely small thickness (typically, the thickness is0.1-atomic-layer thick or larger and 20-atomic-layer thick or smaller),the interface state might affect the second oxide semiconductor layer106 depending on the bonding state between the first oxide semiconductorlayer 104 and the second oxide semiconductor layer 106.

In a transistor employing the band structure illustrated in FIG. 3A, thechannel is less likely to be affected by the interface state andon-state current is less likely to be decreased by the interface state.This is because a channel is formed in a region of the second oxidesemiconductor layer 106 which is far from the interface with the firstoxide semiconductor layer 104 and has the highest electron affinity.Therefore, the transistor can have high on-state current and low Svalue. Furthermore, a difference in energy at the bottom of theconduction band exists between the channel and the interface between thefirst insulating layer 102 and the first oxide semiconductor layer 104at which an interface state might be formed; thus, carriers are noteasily trapped in the interface state. Thus, change in the electricalcharacteristics due to the interface state does not easily occur, sothat the transistor can have high reliability.

The band structure of the second oxide semiconductor layer 106 in FIG.3B has continuous junction near an interface with the second insulatinglayer 110. Specifically, near the interface with the second insulatinglayer 110, the electron affinity becomes larger as the bottom of theconduction band of the second oxide semiconductor layer 106 is fartherfrom the second insulating layer 110. Near the interface with the firstoxide semiconductor layer 104, energy at the bottom of the conductionband of the second oxide semiconductor layer 106 is constant, and theelectron affinity is also constant.

For example, in the case where an In-M-Zn oxide layer (M is Al, Ti, Ga,Y, Zr, La, Ce, Nd, or Hf) is formed as the second oxide semiconductorlayer 106, the second oxide semiconductor layer 106 has a concentrationgradient such that, near the interface with the second insulating layer110, the concentration of the element M is lower as the second oxidesemiconductor layer 106 is farther from the second insulating layer 110,and near the interface with the first oxide semiconductor layer 104, theconcentration of the element M is constant (or substantially constant).In that case, the band structure of the second oxide semiconductor layer106 illustrated in FIG. 3B might be formed.

In the case where an insulating layer including a constituent element(e.g., silicon) which is different from that of an oxide semiconductoris provided as the second insulating layer 110, an interface state dueto junction of different kinds of materials, entry of an impurity, orthe like might be formed at the interface with the second oxidesemiconductor layer 106, like at the interface between the firstinsulating layer 102 and the first oxide semiconductor layer 104. In thecase where the second insulating layer is formed by a method usingplasma (e.g., a plasma CVD method or a sputtering method), an interfacestate due to plasma damage might be formed in the second insulatinglayer 110 or near the interface between the second insulating layer 110and the second oxide semiconductor layer 106.

In a transistor employing the band structure illustrated in FIG. 3B, thechannel is less likely to be affected by the interface state andon-state current is less likely to be decreased by the interface state.This is because a channel can be formed in a region of the second oxidesemiconductor layer 106 which is far from the interface with the secondinsulating layer 110 and has the highest electron affinity. Therefore,the transistor can have high on-state current and low S value.Furthermore, a difference in energy at the bottom of the conduction bandexists between the channel and the interface between the secondinsulating layer 110 and the second oxide semiconductor layer 106 atwhich an interface state might be formed; thus, carriers are not easilytrapped in the interface state. Thus, change in the electricalcharacteristics due to the interface state does not easily occur, sothat the transistor can have high reliability.

The band structure of the second oxide semiconductor layer 106 in FIG.3C has continuous junction near an interface with the first oxidesemiconductor layer 104 and near an interface with the second insulatinglayer 110. Specifically, near the interface with the first oxidesemiconductor layer 104, the electron affinity becomes larger as thebottom of the conduction band of the second oxide semiconductor layer106 is farther from the first oxide semiconductor layer 104; near theinterface with the second insulating layer 110, the electron affinitybecomes larger as the bottom of the conduction band of the second oxidesemiconductor layer 106 is farther from the second insulating layer 110.

For example, in the case where an In-M-Zn oxide layer (M is Al, Ti, Ga,Y, Zr, La, Ce, Nd, or Hf) is formed as the second oxide semiconductorlayer 106, the second oxide semiconductor layer 106 has a concentrationgradient such that, near the interface with the second insulating layer110, the concentration of the element M is lower as the second oxidesemiconductor layer 106 is farther from the second insulating layer 110,and also near the interface with the first oxide semiconductor layer104, the concentration of the element M is lower as the second oxidesemiconductor layer 106 is farther from the first oxide semiconductorlayer 104. In that case, the band structure of the second oxidesemiconductor layer 106 illustrated in FIG. 3C might be formed.

In a transistor employing the band structure illustrated in FIG. 3C, thechannel in the second oxide semiconductor layer 106 is formed in aregion that is apart from an interface states that might occur at theinterfaces (the interface between the first insulating layer 102 and thefirst oxide semiconductor layer 104 and the interface between the secondoxide semiconductor layer 106 and the second insulating layer 110).Therefore, the transistor can have high on-state current and low Svalue. Thus, change in the electrical characteristics due to theinterface state does not easily occur, so that the transistor can havehigh reliability.

Note that the band structure of the stacked structure described in thisembodiment is not limited to the band structures illustrated in FIGS. 3Ato 3C. The composition and electron affinity of the second oxidesemiconductor layer 106 can be changed as appropriate as long as thesecond oxide semiconductor layer 106 has a region whose electronaffinity is higher than that of the first oxide semiconductor layer 104.For example, a band structure may be employed in which the electronaffinity of the second oxide semiconductor layer 106 becomes higher asthe distance from a layer in contact with the second oxide semiconductorlayer 106 (the first oxide semiconductor layer 104 and/or the secondinsulating layer 110) is smaller. In the case where this band structureis employed for a transistor and the electron affinity on theback-channel side is high, high on-state current can be obtained at lowgate voltage; thus, the power consumption of the transistor can be low.Alternatively, when the electron affinity on the front-channel side ishigh, the transistor is sensitive to an electric field from a gateelectrode; thus, on/off of the transistor can be quickly switched andthe transistor can have low S value.

In the case of a band structure in which the electron affinity of thesecond oxide semiconductor layer 106 becomes higher as the distance froma layer in contact with the second oxide semiconductor layer 106 (thefirst oxide semiconductor layer 104 and/or the second insulating layer110) is larger or smaller, the band structure may have a shoulder peak.

In the second oxide semiconductor layer 106, a constituent element of aregion in contact with the first oxide semiconductor layer 104 may bedifferent from a constituent element of a region in contact with thesecond insulating layer 110. In this case, the second oxidesemiconductor layer 106 may have a stacked structure of layers havingdifferent constituent elements.

The oxide semiconductor layer of one embodiment of the present inventionobtained by the above method is an oxide semiconductor layer in whichplasma damage is prevented and the number of defects is reduced. Withuse of the oxide semiconductor layer for a channel of, for example, asemiconductor device, change in the electrical characteristics of thesemiconductor device can be suppressed and the reliability thereof canbe improved.

In the method of forming an oxide semiconductor layer of one embodimentof the present invention, the second oxide semiconductor layer can havehigher crystallinity in the case where an oxide semiconductor materialcontaining a single metal element as a constituent element is used forthe first oxide semiconductor layer serving as a seed crystal, than inthe case where the second oxide semiconductor layer has a single layerstructure of an oxide semiconductor material containing two or moremetal elements as constituent elements. Furthermore, the first oxidesemiconductor layer, which is used as a seed crystal, is relatively easyto crystallize; thus, in the case where deposition is performed whilethe substrate is being heated, a substrate over which the oxidesemiconductor layer having a crystal part is formed is not limited to asingle crystal substrate and may be a substrate having an amorphous orpolycrystalline surface.

Moreover, in the method of forming an oxide semiconductor layer, whichis one embodiment of the present invention, deposition proceedssimultaneously with crystallization; thus, high temperature (e.g.,higher than 1300° C.) heat treatment for the purpose of crystallizationdoes not need to be performed after the formation of the first oxidesemiconductor layer and/or the formation of the second oxidesemiconductor layer. To perform heat treatment at a temperature higherthan 1300° C., a muffle furnace provided with a ceramic partition wallneeds to be used, for example, but such a furnace has the followingproblems: the productivity cannot be increased because it is difficultto increase the size of the furnace, and contamination to a substrate tobe processed might be caused because it is difficult to keep the furnaceclean. Therefore, the formation method of one embodiment of the presentinvention is effective.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

(Embodiment 2)

In this embodiment, described will be specific examples of amanufacturing apparatus with which an oxide semiconductor layer and thelike of one embodiment of the present invention can be formed.

A manufacturing apparatus illustrated in FIG. 4A includes at least aload chamber 302, a transfer chamber 310, a pretreatment chamber 303, atreatment chamber 331 which is a deposition chamber using a thermal CVDmethod, and an unload chamber 306. With the manufacturing apparatusillustrated in FIG. 4A, films can be formed successively without beingexposed to the air. Therefore, in the case where a stacked structure isformed, an impurity can be prevented from entering a film or aninterface between films. Note that in order to prevent, for example,attachment of moisture to inner walls of chambers (including a loadchamber, a treatment chamber, a transfer chamber, a deposition chamber,and an unload chamber) of the manufacturing apparatus, the chambers arepreferably filled with an inert gas (e.g., a nitrogen gas or a rare gas)whose dew point is lower than −60° C., preferably lower than −80° C.,more preferably lower than −100° C. The pressure in the cambers isreduced to less than 1 Pa, preferably less than 0.1 Pa, more preferablyless than 1×10⁻⁴ Pa.

Like the treatment chamber 331, the treatment chamber 304 and/or atreatment chamber 305 may be a chamber using a thermal CVD method (alsoreferred to as a thermal CVD apparatus).

For example, the first oxide semiconductor layer and the second oxidesemiconductor layer are formed in the treatment chamber 331, theinsulating layer is formed in the treatment chamber 304, and aconductive film is formed in the treatment chamber 305. In that case, astacked structure of these layers can be formed successively withoutbeing exposed to the air.

First, a substrate is transferred to the load chamber 302. Next, thesubstrate is transferred to the pretreatment chamber 303 by a transferunit 307 of the transfer chamber 310. In the pretreatment chamber 303,treatment for cleaning the substrate or heat treatment is performed.Then, the substrate is transferred to the treatment chamber 331 and thefirst oxide semiconductor layer and the second oxide semiconductor layerare formed. By the treatment in the pretreatment chamber 303, a surfaceof the substrate can be cleaned. Furthermore, the substrate and thelayers are not exposed to the air in the process from the surfacetreatment of the substrate to the formation of the second oxidesemiconductor layer, so that attachment of an impurity or the like tothe substrate surface can be prevented.

Next, the substrate is transferred to the treatment chamber 304 by thetransfer unit 307, and an insulating layer such as a hafnium oxide layeris formed. The substrate is transferred to the treatment chamber 305 bythe transfer unit 307 and a conductive film such as a tungsten film isformed. Then, the substrate is transferred to the unload chamber 306 bythe transfer unit 307. Through the above steps, the first oxidesemiconductor layer, the second oxide semiconductor layer, theinsulating layer, and the conductive layer can be stacked in this order.

FIG. 4B illustrates an example of a thermal CVD apparatus. In thethermal CVD apparatus, oxidizer (e.g., O₂ or O₃), one kind or pluralkinds of source gases, and the like are supplied at the same time to achamber to which a substrate 320 is transferred; oxidizer, the gases,and the like react with one another near the substrate or at the surfaceof the substrate; and reaction products are deposited to form a film.

The treatment chamber 331 of the thermal CVD apparatus includes at leasta substrate holder 319, a member 321 to which a plurality of source gasinlets is connected, and an exhaust unit 318. Each of the source gasinlets is connected to a source material supply portion (a sourcematerial supply portion 323, 325, 327, or 329) through a supply tube, apressure regulator, a valve, and a flow controller (a flow controller322, 324, 326, or 328). A gas outlet is connected to the exhaust unit318 through a supply tube valve or a pressure regulator.

The atmosphere in the treatment chamber 331 during deposition may be anair atmosphere or a reduced-pressure atmosphere.

When a source gas is supplied, the source gas may be supplied through aplurality of openings formed like openings of a shower head.

To make the thickness of a film on the substrate uniform, the substrate320 fixed on the substrate holder 319 may be rotated by rotating thesubstrate holder 319.

Since the thermal CVD method does not use plasma, a defect due to plasmais not formed in a film.

By the thermal CVD method, various films such as a conductive layer, asemiconductor layer, and an insulating layer can be formed. For example,in the case where an In—Ga—Zn oxide layer is formed, trimethylindium((CH₃)₃In), trimethylgallium ((CH₃)₃Ga), dimethylzinc ((CH₃)₂Zn), andthe like are used as source gases. Note that the combination of sourcegases for the In—Ga—Zn oxide film is not limited to the above. Forexample, triethylgallium ((C₂H₅)₃Ga) can be used instead oftrimethylgallium, and diethylzine ((C₂H₅)₂Zn) can be used instead ofdimethylzinc.

Although FIG. 4A illustrates an example of the multi-chambermanufacturing apparatus having the transfer chamber 310 whose top viewis hexagonal, the top view of the transfer chamber 310 may be polygonal(e.g., heptagonal or octagonal) and more chambers may be connected tothe transfer chamber 310. Alternatively, the top view of the transferchamber 310 of the manufacturing apparatus may be pentagonal ortetragonal. Further alternatively, an in-line manufacturing apparatus inwhich the transfer chamber is omitted by connecting a plurality ofchambers to each other may be used. The in-line manufacturing apparatushas high productivity because there is no transfer chamber and thetransfer time can be shortened. Although FIG. 4A illustrates an exampleof the single wafer manufacturing apparatus, a batch-type depositionapparatus in which films are deposited over a plurality of substrates ata time may be used. In addition, a mechanism for cleaning (such asplasma cleaning) may be included in each treatment chamber.

In FIG. 4A, a thermal CVD apparatus is used as each of the treatmentchambers 304, 305, and 331, but one of the treatment chambers may beanother deposition apparatus such as a sputtering apparatus or an ALDapparatus.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

(Embodiment 3)

In this embodiment, the structure of a transistor of one embodiment ofthe present invention having the stacked structure described inEmbodiment 1 will be described.

<Transistor Structure 1>

FIGS. 5A and 5B are a plan view and a cross-sectional view illustratinga transistor 450 of one embodiment of the present invention. FIG. 5A isa plan view and FIG. 5B is a cross-sectional view taken alongdashed-dotted line A1-A2 and dashed-dotted line A3-A4 in FIG. 5A. Notethat for simplification of the drawing, some components are notillustrated in the plan view in FIG. 5A.

The transistor 450 illustrated in FIGS. 5A and 5B includes an insulatinglayer 402 having a projecting portion over a substrate 400; a firstoxide semiconductor layer 404 and a second oxide semiconductor layer 406over the projecting portion of the insulating layer 402; a sourceelectrode layer 408 a and a drain electrode layer 408 b which are incontact with side surfaces of the first oxide semiconductor layer 404and a top surface and side surfaces of the second oxide semiconductorlayer 406; an insulating layer 410 in contact with the second oxidesemiconductor layer 406 over the source electrode layer 408 a and thedrain electrode layer 408 b; and a gate electrode layer 412 which is incontact with a top surface of the insulating layer 410 and faces theside surfaces of the first oxide semiconductor layer 404 and the topsurface and the side surfaces of the second oxide semiconductor layer406. Note that an insulating layer 414 over the source electrode layer408 a, the drain electrode layer 408 b, and the gate electrode layer 412may be regarded as a component of the transistor 450. The insulatinglayer 402 does not necessarily have a projecting portion.

Note that at least part (or all) of the source electrode layer 408 a(and/or the drain electrode layer 408 b) is provided on at least part(or all) of a surface, side surfaces, a top surface, and/or a bottomsurface of a semiconductor layer such as the second oxide semiconductorlayer 406 (and/or the first oxide semiconductor layer 404).

Alternatively, at least part (or all) of the source electrode layer 408a (and/or the drain electrode layer 408 b) is in contact with at leastpart (or all) of a surface, side surfaces, a top surface, and/or abottom surface of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404). Alternatively, at least part (or all) of the source electrodelayer 408 a (and/or the drain electrode layer 408 b) is in contact withat least part (or all) of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404).

Alternatively, at least part (or all) of the source electrode layer 408a (and/or the drain electrode layer 408 b) is electrically connected toat least part (or all) of a surface, side surfaces, a top surface,and/or a bottom surface of a semiconductor layer such as the secondoxide semiconductor layer 406 (and/or the first oxide semiconductorlayer 404). Alternatively, at least part (or all) of the sourceelectrode layer 408 a (and/or the drain electrode layer 408 b) iselectrically connected to part (or all) of a semiconductor layer such asthe second oxide semiconductor layer 406 (and/or the first oxidesemiconductor layer 404).

Alternatively, at least part (or all) of the source electrode layer 408a (and/or the drain electrode layer 408 b) is provided near at leastpart (or all) of a surface, side surfaces, a top surface, and/or abottom surface of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404). Alternatively, at least part (or all) of the source electrodelayer 408 a (and/or the drain electrode layer 408 b) is provided nearpart (or all) of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404).

Alternatively, at least part (or all) of the source electrode layer 408a (and/or the drain electrode layer 408 b) is provided on a side of atleast part (or all) of a surface, side surfaces, a top surface, and/or abottom surface of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404). Alternatively, at least part (or all) of the source electrodelayer 408 a (and/or the drain electrode layer 408 b) is in provided on aside of part (or all) of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404).

Alternatively, at least part (or all) of the source electrode layer 408a (and/or the drain electrode layer 408 b) is provided obliquely aboveat least part (or all) of a surface, side surfaces, a top surface,and/or a bottom surface of a semiconductor layer such as the secondoxide semiconductor layer 406 (and/or the first oxide semiconductorlayer 404). Alternatively, at least part (or all) of the sourceelectrode layer 408 a (and/or the drain electrode layer 408 b) isprovided obliquely above part (or all) of a semiconductor layer such asthe second oxide semiconductor layer 406 (and/or the first oxidesemiconductor layer 404).

Alternatively, at least part (or all) of the source electrode layer 408a (and/or the drain electrode layer 408 b) is provided above at leastpart (or all) of a surface, side surfaces, a top surface, and/or abottom surface of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404). Alternatively, at least part (or all) of the source electrodelayer 408 a (and/or the drain electrode layer 408 b) is provided abovepart (or all) of a semiconductor layer such as the second oxidesemiconductor layer 406 (and/or the first oxide semiconductor layer404).

In the transistor 450 illustrated in FIGS. 5A and 5B, the insulatinglayer 402 corresponds to the first insulating layer 102 in Embodiment 1,the first oxide semiconductor layer 404 corresponds to the first oxidesemiconductor layer 104 in Embodiment 1, the second oxide semiconductorlayer 406 corresponds to the second oxide semiconductor layer 106 inEmbodiment 1, and the insulating layer 410 functioning as a gateinsulating layer corresponds to the second insulating layer 110 inEmbodiment 1. Therefore, in the transistor 450, the second oxidesemiconductor layer 406 has a crystal part resulting from epitaxialgrowth using the first oxide semiconductor layer 404 as a seed crystal,and a region whose electron affinity is higher than that of the firstoxide semiconductor layer 404. In other words, in the transistor 450,the second oxide semiconductor layer 406 functions as a main currentpath (channel). The second oxide semiconductor layer 406 is an oxidesemiconductor layer in which plasma damage is prevented and the numberof defects is reduced.

As in FIG. 5B, side surfaces of the source electrode layer 408 a and thedrain electrode layer 408 b are in contact with the side surfaces of thesecond oxide semiconductor layer 406 in which a channel is formed. In across section in the channel width direction, the gate electrode layer412 is provided to face the top surface and the side surfaces of thesecond oxide semiconductor layer 406, and the second oxide semiconductorlayer 406 can be electrically surrounded by an electric field of thegate electrode layer 412. Here, a structure of a transistor in which achannel is electrically surrounded by the electric field of the gateelectrode layer 412 (or an electric field of the second oxidesemiconductor layer 406 in which a channel is formed) is referred to asa surrounded channel (s-channel) structure. The transistor 450 has thes-channel structure, which enables to form a channel in the entiresecond oxide semiconductor layer 406 (bulk). In the s-channel structure,a large amount of current can flow between a source and a drain of atransistor, so that a high on-state current can be obtained.

The s-channel structure is suitable for a miniaturized transistorbecause a high on-state current can be obtained. A semiconductor deviceincluding the miniaturized transistor can have a high integration degreeand high density. For example, the channel length of the transistor ispreferably less than or equal to 40 nm, more preferably less than orequal to 30 nm, still more preferably less than or equal to 20 nm andthe channel width of the transistor is preferably less than or equal to40 nm, more preferably less than or equal to 30 nm, still morepreferably less than or equal to 20 nm.

Note that a channel length refers to a distance between a source (asource region or a source electrode) and a drain (a drain region or adrain electrode) in a region where a semiconductor layer and a gateelectrode layer overlap with each other in a plan view. Accordingly, inFIG. 5A, a channel length is a distance between the source electrodelayer 408 a and the drain electrode layer 408 b in a region where thesecond oxide semiconductor layer 406 and the gate electrode layer 412overlap with each other. A channel width refers to the width of a sourceor a drain in a region where a semiconductor layer and a gate electrodelayer overlap with each other. Accordingly, in FIG. 5A, a channel widthis a width of the source electrode layer 408 a or the drain electrodelayer 408 b in a region where the second oxide semiconductor layer 406and the gate electrode layer 412 overlap with each other.

The descriptions on the substrate 100, the first insulating layer 102,the first oxide semiconductor layer 104, the second oxide semiconductorlayer 106, and the second insulating layer 110 can be referred to forthe substrate 400, the insulating layer 402, the first oxidesemiconductor layer 404, the second oxide semiconductor layer 406, andthe insulating layer 410, respectively.

As the source electrode layer 408 a and the drain electrode layer 408 billustrated, a conductive layer capable of extracting oxygen from theoxide semiconductor layer is preferably used. As an example of theconductive layer capable of extracting oxygen from the oxidesemiconductor layer, a conductive layer containing aluminum, titanium,chromium, nickel, molybdenum, tantalum, tungsten, or the like can begiven.

By the conductive layer capable of extracting oxygen from the oxidesemiconductor layer, oxygen in the first oxide semiconductor layer 104and/or the second oxide semiconductor layer 106 is released to formoxygen vacancies in the oxide semiconductor layer in some cases. Oxygenis more likely to be extracted as the temperature is higher. Since theprocess of manufacturing a transistor involves some heat treatmentsteps, oxygen vacancies are likely to be formed in a region of the oxidesemiconductor layer, which is in contact with the source electrode layeror the drain electrode layer. Furthermore, hydrogen enters sites ofoxygen vacancies by heating, and thus the oxide semiconductor layerbecomes n-type in some cases. Thus, due to the source electrode layerand the drain electrode layer, the resistance of a region where theoxide semiconductor layer is in contact with the source electrode layeror the drain electrode layer is reduced, so that the on-state resistanceof the transistor can be reduced.

In the case where a transistor with a short channel length (e.g., lessthan or equal to 200 nm, or less than or equal to 100 nm) ismanufactured, a source and a drain might be short-circuited due toformation of an n-type region. Therefore, in the case where a transistorwith a short channel length is manufactured, a conductive layer capableof moderately extracting oxygen from the oxide semiconductor layer maybe used as the source electrode layer and the drain electrode layer. Asthe conductive layer capable of moderately extracting oxygen, aconductive layer containing nickel, molybdenum, or tungsten can be used,for example.

Furthermore, in the case where a transistor with an extremely shortchannel length (less than or equal to 40 nm, or less than or equal to 30nm) is manufactured, a conductive layer which hardly extracts oxygenfrom the oxide semiconductor layer may be used as the source electrodelayer 408 a and the drain electrode layer 408 b. As the conductive layerwhich hardly extracts oxygen from the oxide semiconductor layer, aconductive layer containing tantalum nitride, titanium nitride, orruthenium can be used, for example. Note that plural kinds of conductivelayers may be stacked.

The gate electrode layer 412 may be formed using a conductive layercontaining one or more of aluminum, titanium, chromium, cobalt, nickel,copper, yttrium, zirconium, molybdenum, ruthenium, silver, tantalum,tungsten, and the like.

As the insulating layer 414, for example, a single layer or a stackedlayer of an insulating layer containing aluminum oxide, magnesium oxide,silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide maybe used.

An oxide semiconductor that can be used for the second oxidesemiconductor layer 406 in which a channel is formed will be describedbelow.

In the case where an oxide semiconductor containing two or more metalelements as constituent elements is used for the second oxidesemiconductor layer 406, the oxide semiconductor preferably contains atleast indium. An oxide can have high carrier mobility (electronmobility) by containing indium, for example. More preferably, the oxidesemiconductor layer contains the element M. The element M is preferablyaluminum, gallium, yttrium, tin, or the like. Other elements that can beused as the element M are, for example, titanium, zirconium, lanthanum,cerium, neodymium, and hafnium. Note that two or more of the aboveelements may be used in combination as the element M. The element M isan element having a high bonding energy with oxygen, for example. Theelement M is an element that can increase the energy gap of the oxide,for example. Furthermore, the oxide semiconductor layer preferablycontains zinc. When the oxide contains zinc, the oxide is likely to becrystallized, for example.

Note that the oxide contained in the second oxide semiconductor layer406 is not limited to the oxide containing indium. The second oxidesemiconductor layer 406 may contain, for example, zinc tin oxide orgallium tin oxide.

An oxide with a wide energy gap is used for the second oxidesemiconductor layer 406. The energy gap of the second oxidesemiconductor layer 406 is, for example, 2.5 eV or larger and 4.2 eV orsmaller, preferably 2.8 eV or larger and 3.8 eV or smaller, morepreferably 3 eV or larger and 3.5 eV or smaller.

An influence of impurities in the oxide semiconductor layer where achannel is formed is described below. To obtain stable electricalcharacteristics of a transistor, it is effective to reduce theconcentration of impurities in the second oxide semiconductor layer 406(including an interface) so that the second oxide semiconductor layer406 has a lower carrier density and is highly purified. The carrierdensity of the second oxide semiconductor layer 406 is lower than1×10¹⁷/cm³, lower than 1×10¹⁵/cm³, or lower than 1×10¹³/cm³. To reducethe concentration of impurities in the second oxide semiconductor layer406, the concentrations of impurities in adjacent layers are preferablyreduced.

For example, silicon in the second oxide semiconductor layer 406 mightserve as a carrier trap or a carrier generation source. Theconcentration of silicon in a region between the second oxidesemiconductor layer 406 and the insulating layer 410 measured by SIMS islower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³,more preferably lower than 2×10¹⁸ atoms/cm³.

When hydrogen is contained in the second oxide semiconductor layer 406,the carrier density is increased in some cases. Thus, the concentrationof hydrogen in the second oxide semiconductor layer 406, measured bySIMS is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower thanor equal to 5×10¹⁹ atoms/cm³, more preferably lower than or equal to1×10¹⁹ atoms/cm³, further more preferably lower than or equal to 5×10¹⁸atoms/cm³. When nitrogen is contained in the second oxide semiconductorlayer 406, the carrier density is increased in some cases. Theconcentration of nitrogen in the second oxide semiconductor layer 406measured by SIMS is lower than 5×10¹⁹ atoms/cm³, preferably lower thanor equal to 5×10¹⁸ atoms/cm³, more preferably lower than or equal to1×10¹⁸ atoms/cm³, further more preferably lower than or equal to 5×10¹⁷atoms/cm³.

The thickness of the first oxide semiconductor layer 404 is extremelysmall; thus, an impurity contained in the first oxide semiconductorlayer 404 might affect the channel. Therefore, the siliconconcentration, the hydrogen concentration, and the nitrogenconcentration in the first oxide semiconductor layer 404 are preferablyreduced as well as those in the second oxide semiconductor layer 406.

To reduce the hydrogen concentration in the first oxide semiconductorlayer 404 and the second oxide semiconductor layer 406, it is preferableto reduce the hydrogen concentration in the insulating layer 402 and theinsulating layer 410. The hydrogen concentration in the insulating layer402 and the insulating layer 410 measured by SIMS is 2×10²⁰ atoms/cm³ orlower, preferably 5×10¹⁹ atoms/cm³ or lower, more preferably 1×10¹⁹atoms/cm³ or lower, still more preferably 5×10¹⁸ atoms/cm³ or lower. Toreduce the nitrogen concentration in the first oxide semiconductor layer404 and the second oxide semiconductor layer 406, it is preferable toreduce the nitrogen concentration in the insulating layer 402 and theinsulating layer 410. The nitrogen concentration in the insulating layer402 and the insulating layer 410 measured by SIMS is lower than 5×10¹⁹atoms/cm³, preferably 5×10¹⁸ atoms/cm³ or lower, more preferably 1×10¹⁸atoms/cm³ or lower, still more preferably 5×10¹⁷ atoms/cm³ or lower.

Next, a crystal part included in the first oxide semiconductor layer 104or the second oxide semiconductor layer 406 will be described.

An oxide semiconductor layer is classified into a single crystal oxidesemiconductor layer and a non-single-crystal oxide semiconductor layer.Examples of a non-single-crystal oxide semiconductor layer include ac-axis aligned crystalline oxide semiconductor (CAAC-OS) layer, apolycrystalline oxide semiconductor layer, a microcrystalline oxidesemiconductor layer, and an amorphous oxide semiconductor layer.

From another perspective, an oxide semiconductor layer is classifiedinto an amorphous oxide semiconductor layer and a crystalline oxidesemiconductor layer. Examples of a crystalline oxide semiconductor layerinclude a single crystal oxide semiconductor layer, a CAAC-OS layer, apolycrystalline oxide semiconductor layer, and a microcrystalline oxidesemiconductor layer.

<CAAC-OS Layer>

First, a CAAC-OS layer is described. Note that a CAAC-OS layer can bereferred to as an oxide semiconductor layer including c-axis alignednanocrystals (CANC).

A CAAC-OS layer is one of oxide semiconductor layers having a pluralityof c-axis aligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OSlayer, which is obtained using a transmission electron microscope (TEM),a plurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS layer, a reduction in electronmobility due to the grain boundary is less likely to occur.

The CAAC-OS layer observed with a TEM is described below. FIG. 20A showsan example of a high-resolution TEM image of a cross section of theCAAC-OS layer which is observed from a direction substantially parallelto the sample surface. The high-resolution TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image. TheCs-corrected high-resolution TEM image can be obtained with, forexample, an atomic resolution analytical electron microscope JEM-ARM200Fmanufactured by JEOL Ltd.

FIG. 20B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 20A. FIG. 20B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS layer isformed (hereinafter, the surface is referred to as a formation surface)or a top surface of the CAAC-OS layer, and is arranged parallel to theformation surface or the top surface of the CAAC-OS.

As shown in FIG. 20B, the CAAC-OS layer has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 20C. FIGS. 20B and 20C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canbe referred to as a nanocrystal (nc).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS layer over asubstrate 5120 is illustrated by such a structure in which bricks orblocks are stacked (see FIG. 20D). The part in which the pellets aretilted as observed in FIG. 20C corresponds to a region 5161 shown inFIG. 20D.

FIG. 21A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS layer observed from a direction substantially perpendicularto the sample surface. FIGS. 21B, 21C, and 21D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 21A,respectively. FIGS. 21B, 21C, and 21D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, the CAAC-OS layer analyzed by X-ray diffraction (XRD) isdescribed. For example, when the structure of a CAAC-OS layer includingan InGaZnO₄ crystal is analyzed by an out-of-plane method, a peakappears at a diffraction angle (2θ) of around 31° as shown in FIG. 22A.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS layer have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS layer.

Note that in structural analysis of the CAAC-OS layer by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak at 2θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS layer. It is preferable that in the CAAC-OS layer analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS layer by anin-plane method in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (φ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (φ axis), as shown in FIG. 22B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor layer of InGaZnO₄, when φ scan is performedwith 2θ fixed at around 56°, as shown in FIG. 22C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are different in the CAAC-OS layer.

Next, the CAAC-OS layer analyzed by electron diffraction is described.For example, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS layer including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 23A might be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OSlayer have c-axis alignment and that the c-axes are aligned in adirection substantially perpendicular to the formation surface or thetop surface of the CAAC-OS layer. Meanwhile, FIG. 23B shows adiffraction pattern obtained in such a manner that an electron beam witha probe diameter of 300 nm is incident on the same sample in a directionperpendicular to the sample surface. As shown in FIG. 23B, a ring-likediffraction pattern is observed. Thus, the electron diffraction alsoindicates that the a-axes and b-axes of the pellets included in theCAAC-OS layer do not have regular alignment. The first ring in FIG. 23Bis considered to be derived from the (010) plane, the (100) plane, andthe like of the InGaZnO₄ crystal. The second ring in FIG. 23B isconsidered to be derived from the (110) plane and the like.

Moreover, the CAAC-OS layer is an oxide semiconductor layer having a lowdensity of defect states. Defects in the oxide semiconductor layer are,for example, a defect due to impurity and oxygen vacancies. Therefore,the CAAC-OS layer can be regarded as an oxide semiconductor layer with alow impurity concentration, or an oxide semiconductor layer having asmall number of oxygen vacancies.

The impurity contained in the oxide semiconductor layer might serve ascarrier traps or serve as carrier generation sources. In addition,oxygen vacancies in the oxide semiconductor layer might serve as carriertraps or serve as carrier generation sources when hydrogen is capturedtherein.

Note that the impurity means an element other than the main componentsof the oxide semiconductor layer, such as hydrogen, carbon, silicon, ora transition metal element. For example, an element (specifically,silicon or the like) having higher strength of bonding to oxygen than ametal element included in an oxide semiconductor layer extracts oxygenfrom the oxide semiconductor layer, which results in disorder of theatomic arrangement and reduced crystallinity of the oxide semiconductorlayer. A heavy metal such as iron or nickel, argon, carbon dioxide, orthe like has a large atomic radius (or molecular radius), and thusdisturbs the atomic arrangement of the oxide semiconductor layer anddecreases crystallinity.

An oxide semiconductor layer having a low density of defect states (asmall number of oxygen vacancies) can have a low carrier density. Suchan oxide semiconductor layer is referred to as a highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorlayer. A CAAC-OS layer has a low impurity concentration and a lowdensity of defect states. That is, a CAAC-OS layer is likely to behighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor layer. Thus, a transistor including a CAAC-OS layerrarely has negative threshold voltage (is rarely normally on). Thehighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor layer has few carrier traps. An electric chargetrapped by the carrier traps in the oxide semiconductor layer takes along time to be released. The trapped electric charge may behave like afixed electric charge. Thus, the transistor which includes the oxidesemiconductor layer having a high impurity concentration and a highdensity of defect states might have unstable electrical characteristics.However, a transistor including a CAAC-OS layer has small variation inelectrical characteristics and high reliability.

Since the CAAC-OS layer has a low density of defect states, the numberof carries trapped in defect states by light irradiation is small.Therefore, in a transistor using the CAAC-OS layer, change in electricalcharacteristics due to irradiation with visible light or ultravioletlight is small.

<Microcrystalline Oxide Semiconductor Layer>

Next, a microcrystalline oxide semiconductor layer is described.

A microcrystalline oxide semiconductor layer has a region in which acrystal part is observed and a region in which a crystal part is notclearly observed in a high-resolution TEM image. In most cases, the sizeof a crystal part included in the microcrystalline oxide semiconductorlayer is greater than or equal to 1 nm and less than or equal to 100 nm,or greater than or equal to 1 nm and less than or equal to 10 nm. Anoxide semiconductor layer including a nanocrystal (nc) that is amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as a nanocrystalline oxidesemiconductor (nc-OS) layer. In a high-resolution TEM image of the nc-OSlayer, for example, a grain boundary is not clearly observed in somecases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS layer.Therefore, a crystal part of the nc-OS layer may be referred to as apellet in the following description.

In the nc-OS layer, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in the nc-OSlayer. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS layer cannot be distinguished from an amorphousoxide semiconductor layer, depending on an analysis method. For example,when the nc-OS layer is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the size of a pellet, a peak which shows a crystalplane does not appear. Furthermore, a diffraction pattern like a halopattern is observed when the nc-OS layer is subjected to electrondiffraction using an electron beam with a probe diameter (e.g., 50 nm orlarger) that is larger than the size of a pellet (the electrondiffraction is also referred to as selected-area electron diffraction).Meanwhile, spots appear in a nanobeam electron diffraction pattern ofthe nc-OS layer when an electron beam having a probe diameter close toor smaller than the size of a pellet is applied. Moreover, in a nanobeamelectron diffraction pattern of the nc-OS layer, regions with highluminance in a circular (ring) pattern are shown in some cases. Also ina nanobeam electron diffraction pattern of the nc-OS layer, a pluralityof spots is shown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS layer can also be referredto as an oxide semiconductor layer including random aligned nanocrystals(RANC) or an oxide semiconductor layer including non-alignednanocrystals (NANC).

The nc-OS layer is an oxide semiconductor layer that has high regularityas compared with an amorphous oxide semiconductor layer. Therefore, thenc-OS layer is likely to have a lower density of defect states than anamorphous oxide semiconductor layer. Note that there is no regularity ofcrystal orientation between different pellets in the nc-OS. Therefore,the nc-OS layer has a higher density of defect states than the CAAC-OSlayer.

<Amorphous Oxide Semiconductor Layer>

Next, an amorphous oxide semiconductor layer is described.

The amorphous oxide semiconductor layer is an oxide semiconductor layerhaving disordered atomic arrangement and no crystal part and exemplifiedby an oxide semiconductor which exists in an amorphous state as quartz.

In a high-resolution TEM image of the amorphous oxide semiconductorlayer, crystal parts cannot be found.

When the amorphous oxide semiconductor layer is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed whenthe amorphous oxide semiconductor layer is subjected to electrondiffraction. Furthermore, a spot is not observed and only a halo patternappears when the amorphous oxide semiconductor layer is subjected tonanobeam electron diffraction.

There are various understandings of an amorphous structure. For example,a structure whose atomic arrangement does not have ordering at all iscalled a completely amorphous structure. Meanwhile, a structure whichhas ordering until the nearest neighbor atomic distance or thesecond-nearest neighbor atomic distance but does not have long-rangeordering is also called an amorphous structure. Therefore, the strictestdefinition does not permit an oxide semiconductor layer to be called anamorphous oxide semiconductor layer as long as even a negligible degreeof ordering is present in an atomic arrangement. At least an oxidesemiconductor layer having long-term ordering cannot be called anamorphous oxide semiconductor layer. Accordingly, because of thepresence of crystal part, for example, a CAAC-OS layer and an nc-OSlayer cannot be called an amorphous oxide semiconductor layer or acompletely amorphous oxide semiconductor layer.

<Amorphous-Like Oxide Semiconductor Layer>

Note that an oxide semiconductor layer may have a structure havingphysical properties intermediate between the nc-OS layer and theamorphous oxide semiconductor layer. The oxide semiconductor layerhaving such a structure is specifically referred to as an amorphous-likeoxide semiconductor (a-like OS) layer.

In a high-resolution TEM image of the a-like OS layer, a void may beobserved. Furthermore, in the high-resolution TEM image, there are aregion where a crystal part is clearly observed and a region where acrystal part is not observed.

An a-like OS layer has an unstable structure because it includes a void.To verify that an a-like OS layer has an unstable structure as comparedwith a CAAC-OS layer and an nc-OS layer, a change in structure caused byelectron irradiation is described below.

An a-like OS layer, an nc-OS layer, and a CAAC-OS layer are prepared assamples subjected to electron irradiation. Each of the samples is anIn—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 24 shows the change in the average size of crystal parts (at 22points to 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 24 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 24, acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3)in FIG. 24, the average crystal sizes in an nc-OS layer and a CAAC-OSlayer are approximately 1.4 nm and approximately 2.1 nm, respectively,regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS layer isinduced by electron irradiation. In contrast, in the nc-OS layer and theCAAC-OS layer, growth of the crystal part is hardly induced by electronirradiation. Therefore, the a-like OS layer has an unstable structure ascompared with the nc-OS layer and the CAAC-OS layer.

The a-like OS layer has a lower density than the nc-OS layer and theCAAC-OS layer because it includes a void. Specifically, the density ofthe a-like OS layer is higher than or equal to 78.6% and lower than92.3% of the density of the single crystal oxide semiconductor layerhaving the same composition. The density of each of the nc-OS layer andthe CAAC-OS layer is higher than or equal to 92.3% and lower than 100%of the density of the single crystal oxide semiconductor layer havingthe same composition. Note that it is difficult to deposit an oxidesemiconductor layer having a density of lower than 78% of the density ofthe single crystal oxide semiconductor layer.

For example, in the case of an oxide semiconductor layer having anatomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, inthe case of the oxide semiconductor layer having an atomic ratio ofIn:Ga:Zn=1:1:1, the density of the a-like OS layer is higher than orequal to 5.0 g/cm³ and lower than 5.9 g/cm³. For example, in the case ofthe oxide semiconductor layer having an atomic ratio of In:Ga:Zn=1:1:1,the density of each of the nc-OS layer and the CAAC-OS layer is higherthan or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductor layers have various structuresand various properties. Note that an oxide semiconductor layer may be astacked layer including two or more films of an amorphous oxidesemiconductor layer, an a-like OS layer, a microcrystalline oxidesemiconductor layer, and a CAAC-OS layer, for example.

<Method of Manufacturing Transistor>

A method of manufacturing the transistor 450 will be described below.For portions similar to those described in Embodiment 1 or 2. Embodiment1 or 2 can be referred to and repetitive description is omitted in somecases.

First, the substrate 400 is prepared. The substrate 400 can be formed ofa material similar to that of the substrate 100. A substrate alreadyprovided with a semiconductor element may be used as the substrate 400.

Alternatively, a flexible substrate may be used as the substrate 400. Asa method of providing a transistor over a flexible substrate, there is amethod in which the transistor is formed over a non-flexible substrateand then the transistor is separated and transferred to the substrate400 which is a flexible substrate. In that case, a separation layer ispreferably provided between the non-flexible substrate and thetransistor.

The insulating layer 402 is formed over the substrate 400. After theformation of the insulating layer 402, oxygen may be added to theinsulating layer 402 so that the insulating layer contains oxygen inexcess of the stoichiometric composition. Oxygen may be added by plasmatreatment, an ion implantation method, or the like. In the case whereoxygen may be added by an ion implantation method, the accelerationvoltage may be 2 kV or higher and 100 kV or lower and the dose is 5×10¹⁴ions/cm² or more and 5×10¹⁶ ions/cm² or less, for example.

Next, the first oxide semiconductor layer 404 and the second oxidesemiconductor layer 406 are stacked over the insulating layer 402. Then,the layers are processed into an island shape by an etching method usinga photolithography technique. In this etching step, the insulating layer402 may also be etched so that the thickness of a region exposed fromthe first oxide semiconductor layer 404 can be reduced. However, theinsulating layer 402 is moderately etched not to expose the surface ofthe substrate 400. By etching the insulating layer 402 moderately, thegate electrode layer 412 to be formed later can cover the second oxidesemiconductor layer 406 easily. Note that to miniaturize the transistor,a hard mask may be used in processing the first oxide semiconductorlayer 404 and the second oxide semiconductor layer 406.

In the processing of the first oxide semiconductor layer 404 and thesecond oxide semiconductor layer 406, etching is preferably performednot to cause damage to processed surfaces of the first oxidesemiconductor layer 404 and the second oxide semiconductor layer 406.For example, neutral beam etching may be performed by a dry etchingmethod. A neutral beam does not cause buildup of electrical charges andhas low energy, which enables low damage etching. Alternatively, in thecase where the first oxide semiconductor layer 404 and the second oxidesemiconductor layer 406 are crystalline layers, a wet etching method inwhich an etching rate is changed depending on a crystal plane may beused. By using a wet etching method, damage to a processed surface canbe reduced.

First heat treatment may be performed after the second oxidesemiconductor layer 406 is formed. The first heat treatment may beperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 300° C. andlower than or equal to 500° C., in an inert gas atmosphere, anatmosphere containing an oxidizing gas at 10 ppm or more, or a reducedpressure state. Alternatively, the first heat treatment may be performedin such a manner that heat treatment is performed in an inert gasatmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, in order tocompensate desorbed oxygen. By the first heat treatment, thecrystallinity of the second oxide semiconductor layer 406 can beincreased, and in addition, impurities such as hydrogen and water can beremoved from the base insulating layer 402.

Next, a conductive film to be the source electrode layer 408 a and thedrain electrode layer 408 b is formed to cover the second oxidesemiconductor layer 406. The conductive film may be formed by asputtering method, a CVD method, an MBE method, an ALD method, or a PLDmethod. To reduce plasma damage to the first oxide semiconductor layeror the second oxide semiconductor layer, a thermal CVD method such as anMCVD method is preferably used.

Next, the conductive film is divided by etching to form the sourceelectrode layer 408 a and the drain electrode layer 408 b. Note thatwhen the conductive film is etched, end portions of the source electrodelayer 408 a and the drain electrode layer 408 b are rounded (have curvedsurfaces) in some cases. Furthermore, when the conductive film isetched, the insulating layer 402 is etched and a region with a reducedthickness is formed in some cases.

Next, the insulating layer 410 functioning as a gate insulating layer isformed over the second oxide semiconductor layer 406, the sourceelectrode layer 408 a, and the drain electrode layer 408 b. Then, thegate electrode layer 412 is formed over the insulating layer 410. Thegate electrode layer 412 may be formed by a sputtering method, a CVDmethod, an MBE method, an ALD method, or a PLD method. To reduce plasmadamage to the first oxide semiconductor layer or the second oxidesemiconductor layer, a thermal CVD method such as an MCVD method ispreferably used.

Next, the insulating layer 414 is formed over the source electrode layer408 a, the drain electrode layer 408 b, the insulating layer 410, andthe gate electrode layer 412. The insulating layer 414 may be formed bya sputtering method, a CVD method, an MBE method, an ALD method, or aPLD method. To reduce plasma damage to the first oxide semiconductorlayer or the second oxide semiconductor layer, a thermal CVD method oran ALD method is preferably used.

Next, second heat treatment may be performed. The second heat treatmentcan be performed under conditions similar to those of the first heattreatment. The second heat treatment allows reducing oxygen vacancies inthe second oxide semiconductor layer 406 in some cases.

Through the above steps, the transistor 450 illustrated in FIGS. 5A and5B can be manufactured.

Although in FIG. 5B, the gate electrode layer 412 is provided above thefirst oxide semiconductor layer 404 and the second oxide semiconductorlayer 406, one embodiment of the present invention is not limited tothis structure. For example, a gate electrode layer 413 may be providedbelow the first oxide semiconductor layer 404 and the second oxidesemiconductor layer 406, as illustrated in FIG. 17A. A variety ofmaterials can be used for the gate electrode layer 413, as well as forthe gate electrode layer 412. Note that a potential or signal suppliedto the gate electrode layer 413 may be the same as or different from apotential or signal supplied to the gate electrode layer 412. Bysupplying a constant potential to the gate electrode layer 413, thethreshold voltage of the transistor may be controlled. FIG. 17Billustrates an example in which the gate electrode layer 412 iselectrically connected to the gate electrode layer 413 through anopening. Note that in a case other than the case of FIGS. 5A and 5B, thegate electrode layer 413 can be provided in a similar manner.

<Modification Example of Transistor Structure 1>

As in a transistor 460 illustrated in FIGS. 6A and 6B, a third oxidesemiconductor layer 407 may be provided between the insulating layer 410and the second oxide semiconductor layer 406. The same material as thesecond oxide semiconductor layer 406 can be used for the third oxidesemiconductor layer 407. Since the third oxide semiconductor layer 407is in contact with the insulating layer 410 which might contain aconstituent element (e.g., silicon) that is different from that of anoxide semiconductor, an interface state due to junction of differentkinds of materials, entry of an impurity, or the like might be formed atan interface between the third oxide semiconductor layer 407 and theinsulating layer 410. For this reason, to stabilize electricalcharacteristics of a transistor, a channel is preferably formed in thesecond oxide semiconductor layer 406. Thus, a material whose electronaffinity is lower than that of the second oxide semiconductor layer 406is preferably used for the third oxide semiconductor layer 407.

A method without using plasma, in particular, an MOCVD method ispreferably used for forming the third oxide semiconductor layer 407. Inthe case where the third oxide semiconductor layer 407 is formed by anMOCVD method, the third oxide semiconductor layer 407 can have a crystalpart by epitaxial growth using a crystal part in the second oxidesemiconductor layer 406 as a seed crystal. Note that the description ofthe transistor illustrated in FIGS. 5A and 5B is referred to for thestructures of the other components.

Although in FIG. 6B, the gate electrode layer 412 is provided above thethird oxide semiconductor layer 407, one embodiment of the presentinvention is not limited to this structure. For example, a gateelectrode layer 413 may be provided below the first oxide semiconductorlayer 404 to the third oxide semiconductor layer 407, as illustrated inFIG. 18A. FIG. 18B illustrates an example in which the gate electrodelayer 412 is electrically connected to the gate electrode layer 413through an opening. Note that in a case other than the cases of FIGS. 5Aand 5B and FIGS. 6A and 6B, the gate electrode layer 413 can be providedin a similar manner.

<Transistor Structure 2>

FIGS. 7A and 7B are a plan view and a cross-sectional view illustratinga transistor 550 of one embodiment of the present invention. FIG. 7A isa plan view and FIG. 7B is a cross-sectional view taken alongdashed-dotted line B1-B2 and dashed-dotted line B3-B4 in FIG. 7A. Notethat for simplification of the drawing, some components are notillustrated in the plan view in FIG. 7A.

The transistor 550 illustrated in FIGS. 7A and 7B includes an insulatinglayer 502 having a projecting portion over a substrate 500; a firstoxide semiconductor layer 504 and a second oxide semiconductor layer 506over the projecting portion of the insulating layer 502; an insulatinglayer 510 over the second oxide semiconductor layer 506; a gateelectrode layer 512 which is in contact with a top surface of theinsulating layer 510 and faces a top surface and side surfaces of thesecond oxide semiconductor layer 506; an insulating layer 514 which isover the second oxide semiconductor layer 506 and the gate electrodelayer 512 and has openings reaching the second oxide semiconductor layer506; a source electrode layer 516 a and a drain electrode layer 516 bwhich fill the openings; and a conductive layer 518 a and a conductivelayer 518 b which are in contact with the source electrode layer 516 aand the drain electrode layer 516 b, respectively. Note that theinsulating layer 502 does not necessarily have a projecting portion.

In the transistor 550 in FIGS. 7A and 7B, the source electrode layer 516a and the drain electrode layer 516 b are provided so as not to overlapwith the gate electrode layer 512. Thus, parasitic capacitance generatedbetween the gate electrode layer 512 and the source electrode layer 516a or the drain electrode layer 516 b can be reduced. For this reason,the transistor in FIGS. 7A and 7B can have excellent switchingcharacteristics.

In addition, the level of the top surface of the insulating layer 514,that of the source electrode layer 516 a, and that of the drainelectrode layer 516 b are the same; thus, shape defects do not easilyoccur. Therefore, a semiconductor device including the transistor can bemanufactured with high yield.

As for the conductive layer 518 a and the conductive layer 518 b, forexample, a single layer or a stacked layer of a conductive layercontaining one or more of aluminum, titanium, chromium, cobalt, nickel,copper, yttrium, zirconium, molybdenum, ruthenium, silver, tantalum, andtungsten may be used.

The description on the substrate 400, the insulating layer 402, thefirst oxide semiconductor layer 404, the second oxide semiconductorlayer 406, the source electrode layer 408 a and the drain electrodelayer 408 b, the insulating layer 410, the gate electrode layer 412, andthe insulating layer 414 can be referred to for the substrate 500, theinsulating layer 502, the first oxide semiconductor layer 504, thesecond oxide semiconductor layer 506, the source electrode layer 516 aand the drain electrode layer 516 b, the insulating layer 510, the gateelectrode layer 512, and the insulating layer 514, respectively.

Although in FIG. 7B, the gate electrode layer 512 is provided above thefirst oxide semiconductor layer 504 and the second oxide semiconductorlayer 506, one embodiment of the present invention is not limited tothis structure. For example, a gate electrode layer 513 may be providedbelow the first oxide semiconductor layer 504 and the second oxidesemiconductor layer 506, as illustrated in FIG. 19A. A variety ofmaterials can be used for the gate electrode layer 513, as well as forthe gate electrode layer 512. Note that a potential or signal suppliedto the gate electrode layer 513 may be the same as or different from apotential or signal supplied to the gate electrode layer 512. Bysupplying a constant potential to the gate electrode layer 513, thethreshold voltage of the transistor may be controlled. FIG. 19Billustrates an example in which the gate electrode layer 512 iselectrically connected to the gate electrode layer 513 through a contacthole. The gate electrode layer 513 may be provided to overlap with thesource electrode layer 516 a and the drain electrode layer 516 b. Thatcase is illustrated in FIG. 19B. Note that in a case other than thecases of FIGS. 5A and 5B, FIGS. 6A and 6B, and FIGS. 7A and 7B, the gateelectrode layer 513 can be provided in a similar manner.

<Modification Example of Transistor Structure 2>

In the transistor illustrated in FIGS. 7A and 7B, a third oxidesemiconductor layer may be provided between the insulating layer 510 andthe second oxide semiconductor layer 506. The description on the thirdoxide semiconductor layer 407 can be referred to for the third oxidesemiconductor layer. Note that the description on the transistorillustrated in FIGS. 7A and 7B can be referred to for the structures ofthe other components.

<Transistor Structure 3>

FIGS. 8A and 8B are a plan view and a cross-sectional view illustratinga transistor 650 of one embodiment of the present invention. FIG. 8A isa plan view and FIG. 8B is a cross-sectional view taken alongdashed-dotted line C1-C2 and dashed-dotted line C3-C4 in FIG. 8A. Notethat for simplification of the drawing, some components are notillustrated in the plan view in FIG. 8A.

The transistor 650 illustrated in FIGS. 8A and 8B includes a gateelectrode layer 612 over a substrate 600; an insulating layer 602 overthe gate electrode layer 612; a first oxide semiconductor layer 604 overthe insulating layer 602; a second oxide semiconductor layer 606 overthe first oxide semiconductor layer 604; a source electrode layer 608 aand a drain electrode layer 608 b in contact with side surfaces of thefirst oxide semiconductor layer 604 and a top surface and side surfacesof the second oxide semiconductor layer 606; and an insulating layer 610over the second oxide semiconductor layer 606, the source electrodelayer 608 a, and the drain electrode layer 608 b. Note that aninsulating layer may be provided between the substrate 600 and the gateelectrode layer 612.

The transistor 650 may include a conductive layer which overlaps withthe second oxide semiconductor layer 606 with the insulating layer 610provided therebetween. The conductive layer functions as a second gateelectrode layer of the transistor 650. An s-channel structure may beformed using the second gate electrode.

The description on the substrate 400, the gate electrode layer 412, theinsulating layer 402, the first oxide semiconductor layer 404, thesecond oxide semiconductor layer 406, the source electrode layer 408 aand the drain electrode layer 408 b, and the insulating layer 410 can bereferred to for the substrate 600, the gate electrode layer 612, theinsulating layer 602, the first oxide semiconductor layer 604, thesecond oxide semiconductor layer 606, the source electrode layer 608 aand the drain electrode layer 608 b, and the insulating layer 610,respectively.

In each of the above-described transistors of one embodiment of thepresent invention, a channel is formed in an oxide semiconductor layerin which plasma damage is prevented and the number of defects isreduced; thus, change in electrical characteristics of the transistorcan be suppressed. A semiconductor device including the transistor canhave improved reliability.

Although the case where an oxide semiconductor layer is used for achannel and the like is described in this embodiment as an example, oneembodiment of the present invention is not limited thereto. For example,depending on circumstances or conditions, a material containing Si(silicon), Ge (germanium), SiGe (silicon germanium), GaAs (galliumarsenide), or the like may be used for a channel, the vicinity of thechannel, a source region, a drain region, or the like.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

(Embodiment 4)

An example of a semiconductor device of one embodiment of the presentinvention is described in this embodiment.

<Circuit>

An example of a circuit including a transistor of one embodiment of thepresent invention is described below.

[Cross-Sectional Structure]

FIG. 9A is a cross-sectional view of a semiconductor device of oneembodiment of the present invention. The semiconductor deviceillustrated in FIG. 9A includes a transistor 2200 using a firstsemiconductor in a lower portion and a transistor 2100 using a secondsemiconductor in an upper portion. FIG. 9A shows an example in which thetransistor illustrated in FIGS. 5A and 5B is used as the transistor 2100using the second semiconductor. Note that a transistor having adifferent structure described in Embodiment 3 may be used as thetransistor 2100.

As the first semiconductor, a semiconductor having an energy gapdifferent from that of the second semiconductor may be used. Forexample, the first semiconductor may be a semiconductor other than anoxide semiconductor and the second semiconductor may be an oxidesemiconductor. When single crystal silicon is used as the firstsemiconductor, the transistor 2200 capable of high-speed operation canbe obtained. When an oxide semiconductor is used as the secondsemiconductor, the transistor 2100 that is suitable for low off-statecurrent can be obtained.

Note that the transistor 2200 may be either an n-channel transistor or ap-channel transistor, and an appropriate transistor is used inaccordance with a circuit. As the transistor 2100 and/or the transistor2200, the above-described transistor or the transistor illustrated inFIG. 9A is not necessarily used in some cases.

The semiconductor device illustrated in FIG. 9A includes the transistor2100 above the transistor 2200 with an insulating layer 2201 and aninsulating layer 2207 provided therebetween. Between the transistor 2200and the transistor 2100, a plurality of conductive layers 2202 whichfunction as wirings are provided. Wirings or electrodes provided in anupper layer and a lower layer are electrically connected to each otherby a plurality of conductive layers 2203 embedded in insulating films.Furthermore, the semiconductor device includes an insulating layer 2204over the transistor 2100, a conductive film 2205 over the insulatinglayer 2204, and a conductive layer 2206 formed in the same layer(through the same steps) as a source electrode layer and a drainelectrode layer of the transistor 2100.

By stacking a plurality of transistors, a plurality of circuits can bearranged with high density.

Here, in the case where single crystal silicon is used as the firstsemiconductor of the transistor 2200, the hydrogen concentration in aninsulating layer near the first semiconductor of the transistor 2200 ispreferably high. The hydrogen terminates dangling bonds of silicon, sothat the reliability of the transistor 2200 can be increased. On theother hand, the hydrogen concentration in an insulating layer near theoxide semiconductor of the transistor 2100 is preferably low. Thehydrogen causes generation of carriers in the oxide semiconductor, whichmight lead to a decrease in the reliability of the transistor 2100.Therefore, in the case where the transistor 2200 using single crystalsilicon and the transistor 2100 using an oxide semiconductor arestacked, providing the insulating layer 2207 having a function ofblocking hydrogen between the transistors is effective because thereliability of the transistors can be increased.

The insulating layer 2207 may be, for example, formed to have asingle-layer structure or a stacked-layer structure using an insulatinglayer containing aluminum oxide, aluminum oxynitride, gallium oxide,gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide,hafnium oxynitride, yttria-stabilized zirconia (YSZ), or the like.

Furthermore, an insulating layer having a function of blocking hydrogenis preferably formed over the transistor 2100 to cover the transistor2100 using an oxide semiconductor. As the insulating layer, aninsulating layer that is similar to the insulating layer 2207 can beused, and in particular, an aluminum oxide film is preferably used. Thealuminum oxide film has a high blocking effect of preventing penetrationof both oxygen and impurities such as hydrogen and moisture. Thus, byusing the aluminum oxide film as the insulating layer 2208 covering thetransistor 2100, release of oxygen from the oxide semiconductor includedin the transistor 2100 can be prevented and entry of water and hydrogeninto the oxide semiconductor can be prevented.

Note that the transistor 2200 can be a transistor of various typeswithout being limited to a planar type transistor. For example, aFIN-type transistor or a TRI-GATE transistor can be used. An example ofa cross-sectional view in this case is illustrated in FIG. 9D. Aninsulating layer 2212 is provided over a semiconductor substrate 2211.The semiconductor substrate 2211 is processed to have a projectingportion in the channel width direction of the transistor 2200. A gateinsulating film 2214 is provided over the processed semiconductorsubstrate 2211, and a gate electrode 2213 is provided over the gateinsulating film 2214. Source and drain regions 2215 are formed in thesemiconductor substrate 2211. Note that here is shown an example inwhich the semiconductor substrate 2211 is processed to have a projectingportion; however, one embodiment of the present invention is not limitedthereto. For example, a semiconductor region having a projecting portionmay be formed by processing an SOI substrate.

[Circuit Configuration Example]

In the above circuit, electrodes of the transistor 2100 and thetransistor 2200 can be connected in a variety of ways; thus, a varietyof circuits can be formed. Examples of circuit configurations which canbe achieved by using a semiconductor device of one embodiment of thepresent invention are shown below.

[CMOS Circuit]

A circuit diagram in FIG. 9B shows a configuration of a CMOS circuit inwhich the p-channel transistor 2200 and the n-channel transistor 2100are connected to each other in series and in which gates of them areconnected to each other.

[Analog Switch]

A circuit diagram in FIG. 9C shows a configuration in which sources ofthe transistors 2100 and 2200 are connected to each other and drains ofthe transistors 2100 and 2200 are connected to each other. With such aconfiguration, the transistors can function as an analog switch.

[Memory Device Example]

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is shown in FIGS. 10A and 10B.

The semiconductor device illustrated in FIG. 10A includes a transistor3200 using a first semiconductor, a transistor 3300 using a secondsemiconductor, and a capacitor 3400. Note that any of theabove-described transistors described in Embodiment 3 can be used as thetransistor 3300.

The transistor 3300 is a transistor using an oxide semiconductor. Sincethe off-state current of the transistor 3300 is low, stored data can beretained for a long period at a predetermined node of the semiconductordevice. In other words, power consumption of the semiconductor devicecan be reduced because refresh operation becomes unnecessary or thefrequency of refresh operation can be extremely low.

In FIG. 10A, a first wiring 3001 is electrically connected to a sourceof the transistor 3200. A second wiring 3002 is electrically connectedto a drain of the transistor 3200. A third wiring 3003 is electricallyconnected to one of the source and the drain of the transistor 3300. Afourth wiring 3004 is electrically connected to the gate of thetransistor 3300. The gate of the transistor 3200 and the other of thesource and the drain of the transistor 3300 are electrically connectedto the one electrode of the capacitor 3400. A fifth wiring 3005 iselectrically connected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 10A has a feature that the potential ofthe gate of the transistor 3200 can be retained, and thus enableswriting, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 isturned on, so that the transistor 3300 is turned on. Accordingly, thepotential of the third wiring 3003 is supplied to a node FG where thegate of the transistor 3200 and the one electrode of the capacitor 3400are electrically connected to each other. That is, a predeterminedcharge is supplied to the gate of the transistor 3200 (writing). Here,one of two kinds of charges providing different potential levels(hereinafter referred to as a low-level charge and a high-level charge)is supplied. After that, the potential of the fourth wiring 3004 is setto a potential at which the transistor 3300 is turned off, whereby thecharge is held at the node FG (retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the node FG is retained for a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of charge retained in the node FG. This is because in the case ofusing an n-channel transistor as the transistor 3200, an apparentthreshold voltage V_(th) _(_) _(H) at the time when the high-levelcharge is given to the gate of the transistor 3200 is lower than anapparent threshold voltage V_(th) _(_) _(L) at the time when thelow-level charge is given to the gate of the transistor 3200. Here, anapparent threshold voltage refers to the potential of the fifth wiring3005 which is needed to turn on the transistor 3200. Thus, the potentialof the fifth wiring 3005 is set to a potential V₀ which is betweenV_(th) _(_) _(H) and V_(th) _(_) _(L), whereby charge supplied to thenode FG can be determined. For example, in the case where the high-levelcharge is supplied to the node FG in writing and the potential of thefifth wiring 3005 is V₀ (>V_(th) _(_) _(H)), the transistor 3200 isturned on. On the other hand, in the case where the low-level charge issupplied to the node FG in writing, even when the potential of the fifthwiring 3005 is V₀ (<V_(th) _(_) _(L)), the transistor 3200 remains off.Thus, the data retained in the node FG can be read by determining thepotential of the second wiring 3002.

Note that in the case where memory cells are arrayed, it is necessarythat data of a desired memory cell be read in read operation. In thecase where data of the other memory cells is not read, the fifth wiring3005 may be supplied with a potential at which the transistor 3200 isturned off regardless of the charge supplied to the node FG, that is, apotential lower than V_(th) _(_) _(H). Alternatively, the fifth wiring3005 may be supplied with a potential at which the transistor 3200 isturned on regardless of the charge supplied to the node FG, that is, apotential higher than V_(th) _(_) _(L).

The semiconductor device in FIG. 10B is different form the semiconductordevice in FIG. 10A in that the transistor 3200 is not provided. Also inthis case, writing and retaining operation of data can be performed in amanner similar to the semiconductor device in FIG. 10A.

Reading of data in the semiconductor device in FIG. 10B is described.When the transistor 3300 is turned on, the third wiring 3003 which is ina floating state and the capacitor 3400 are electrically connected toeach other, and the charge is redistributed between the third wiring3003 and the capacitor 3400. As a result, the potential of the thirdwiring 3003 is changed. The amount of change in potential of the thirdwiring 3003 varies depending on the potential of the one electrode ofthe capacitor 3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the one electrode of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of a firstterminal of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential ofthe third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor may be usedfor a driver circuit for driving a memory cell, and a transistorincluding the second semiconductor may be stacked over the drivercircuit as the transistor 3300.

When including a transistor using an oxide semiconductor and having anextremely low off-state current, the semiconductor device describedabove can retain stored data for a long time. In other words, refreshoperation becomes unnecessary or the frequency of the refresh operationcan be extremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device, high voltage is not needed forwriting data and deterioration of elements is less likely to occur.Unlike in a conventional nonvolatile memory, for example, it is notnecessary to inject and extract electrons into and from a floating gate;thus, a problem such as deterioration of an insulating layer is notcaused. That is, the semiconductor device of one embodiment of thepresent invention does not have a limit on the number of times data canbe rewritten, which is a problem of a conventional nonvolatile memory,and the reliability thereof is drastically improved. Furthermore, datais written depending on the state of the transistor (on or off), wherebyhigh-speed operation can be easily achieved.

<RF Tag>

An RF tag including the transistor or the memory device is describedbelow with reference to FIG. 11.

The RF tag of one embodiment of the present invention includes a memorycircuit, stores data in the memory circuit, and transmits and receivesdata to/from the outside by using contactless means, for example,wireless communication. With these features, the RF tag can be used foran individual authentication system in which an object or the like isrecognized by reading the individual information, for example. Note thatthe RF tag is required to have high reliability in order to be used forthis purpose.

A configuration of the RF tag will be described with reference to FIG.11. FIG. 11 is a block diagram illustrating a configuration example ofan RF tag.

As shown in FIG. 11, an RF tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RF tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A semiconductor of a transistor having arectifying function included in the demodulation circuit 807 may be amaterial which enables a reverse current to be low enough, for example,an oxide semiconductor. This can suppress the phenomenon of a rectifyingfunction becoming weaker due to generation of a reverse current andprevent saturation of the output from the demodulation circuit. In otherwords, the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RF tag 800.

Next, the structure of each circuit will be described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 805.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 805. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit that generatesa reset signal of the logic circuit 809 by utilizing rise of the stablepower supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 808 performs modulation in accordance with data to beoutput from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate as needed.

Here, the above-described memory device can be used as the memorycircuit 810. Since the memory device of one embodiment of the presentinvention can retain data even when not powered, the memory device issuitable for an RF tag. Further, the memory device of one embodiment ofthe present invention needs power (voltage) needed for data writinglower than that needed in a conventional nonvolatile memory; thus, it ispossible to prevent a difference between the maximum communication rangein data reading and that in data writing. Furthermore, it is possible tosuppress malfunction or incorrect writing which is caused by powershortage in data writing.

Since the memory device of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RF tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

<Application Examples of RF Tag>

Application examples of the RF tag of one embodiment of the presentinvention are shown below with reference to FIGS. 12A to 12F. The RF tagis widely used and can be provided for, for example, products such asbills, coins, securities, bearer bonds, documents (e.g., driver'slicenses or resident's cards, see FIG. 12A), recording media (e.g., DVDsor video tapes, see FIG. 12B), packaging containers (e.g., wrappingpaper or bottles, see FIG. 12C), vehicles (e.g., bicycles, see FIG.12D), personal belongings (e.g., bags or glasses), foods, plants,animals, human bodies, clothing, household goods, medical supplies suchas medicine and chemicals, and electronic devices (e.g., liquid crystaldisplay devices, EL display devices, television sets, or cellularphones), or tags on products (see FIGS. 12E and 12F).

An RF tag 4000 of one embodiment of the present invention is fixed onproducts by, for example, being attached to a surface thereof or beingembedded therein. For example, the RF tag 4000 is fixed to each productby being embedded in paper of a book, or embedded in an organic resin ofa package. The RF tag 4000) of one embodiment of the present inventionis small, thin, and lightweight, so that the design of a product is notimpaired even after the RF tag 4000 of one embodiment of the presentinvention is fixed thereto. Further, bills, coins, securities, bearerbonds, documents, or the like can have identification functions by beingprovided with the RF tag 4000 of one embodiment of the presentinvention, and the identification functions can be utilized to preventcounterfeits. Moreover, the efficiency of a system such as an inspectionsystem can be improved by providing the RF tag 4000 of one embodiment ofthe present invention for packaging containers, recording media,personal belongings, foods, clothing, household goods, electronicdevices, or the like. Vehicles can also have higher security againsttheft or the like by being provided with the RF tag 4000 of oneembodiment of the present invention.

As described above, the RF tag of one embodiment of the presentinvention can be used for the above-described purposes.

<CPU>

A CPU including a semiconductor device such as any of theabove-described transistors or the above-described memory device isdescribed below.

FIG. 13 is a block diagram illustrating a configuration example of a CPUincluding any of the above-described transistors as a component.

The CPU illustrated in FIG. 13 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198 (BusI/F), a rewritable ROM 1199, and an ROM interface 1189 (ROM I/F). Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The rewritable ROM 1199 and the ROMinterface 1189 may be provided over a separate chip. Needless to say,the CPU in FIG. 13 is just an example in which the configuration hasbeen simplified, and an actual CPU may have a variety of configurationsdepending on the application. For example, the CPU may have thefollowing configuration: a structure including the CPU illustrated inFIG. 13 or an arithmetic circuit is considered as one core; a pluralityof the cores are included; and the cores operate in parallel. The numberof bits that the CPU can process in an internal arithmetic circuit or ina data bus can be 8, 16, 32, or 64, for example.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 13, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of theabove-described transistors, the above-described memory device, or thelike can be used.

In the CPU illustrated in FIG. 13, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is retained by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data retaining by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data retaining by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

FIG. 14 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the above-described memory device can be used as the circuit 1202.When supply of a power supply voltage to the memory element 1200 isstopped, GND (0 V) or a potential at which the transistor 1209 in thecircuit 1202 is turned off continues to be input to a gate of thetransistor 1209. For example, the gate of the transistor 1209 isgrounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with the low power supply potential (e.g., GND) or the highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the wiring, orthe like is actively utilized.

A control signal WE is input to a first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 14illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 14, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 14, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a film formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon film or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor layer. Further alternatively, in the memory element 1200,a transistor in which a channel is formed in an oxide semiconductorlayer can be included besides the transistor 1209, and a transistor inwhich a channel is formed in a layer or the substrate 1190 including asemiconductor other than an oxide semiconductor can be used for the restof the transistors.

As the circuit 1201 in FIG. 14, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor layer is extremely low. For example, the off-statecurrent of a transistor in which a channel is formed in an oxidesemiconductor layer is significantly lower than that of a transistor inwhich a channel is formed in silicon having crystallinity. Thus, whenthe transistor is used as the transistor 1209, a signal held in thecapacitor 1208 is retained for a long time also in a period during whichthe power supply voltage is not supplied to the memory element 1200. Thememory element 1200 can accordingly retain the stored content (data)also in a period during which the supply of the power supply voltage isstopped.

Since the above-described memory element performs pre-charge operationwith the switch 1203 and the switch 1204, the time required for thecircuit 1201 to retain original data again after the supply of the powersupply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By applying the above-described memory element 1200 to a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Furthermore, shortly after thesupply of the power supply voltage is restarted, the memory device canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU, the memory element1200 can also be used in an LSI such as a digital signal processor(DSP), a custom LSI, or a programmable logic device (PLD), and a radiofrequency identification (RF).

<Display Panel>

A display panel including a semiconductor device such as any of theabove-described transistors is described below.

FIG. 15A is a top view of the display panel of one embodiment of thepresent invention. FIG. 15B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display panel of one embodiment of the presentinvention. FIG. 15C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display panel of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance withEmbodiment 3. The transistor can easily be an n-channel transistor, andthus, part of a driver circuit that can be formed using an n-channeltransistor in the driver circuit is formed over the same substrate asthe transistor of the pixel portion. With the use of the transistordescribed in Embodiment 3 for the pixel portion or the driver circuit inthis manner, a highly reliable display device can be provided.

FIG. 15A is a block diagram illustrating an example of an active matrixdisplay device. A pixel portion 701, a first scan line driver circuit702, a second scan line driver circuit 703, and a signal line drivercircuit 704 are formed over a substrate 700 of the display device. Inthe pixel portion 701, a plurality of signal lines extended from thesignal line driver circuit 704 are arranged and a plurality of scanlines extended from the first scan line driver circuit 702 and thesecond scan line driver circuit 703 are arranged. Note that pixels whichinclude display elements are arranged in a matrix in regions where thescan lines and the signal lines are crossed. The substrate 700 of thedisplay device is connected to a timing control circuit (also referredto as a controller or a controller IC) through a connection portion suchas a flexible printed circuit (FPC).

In FIG. 15A, the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 areformed over the substrate 700 where the pixel portion 701 is formed.Accordingly, the number of components that are provided outside, such asa driver circuit, can be reduced, so that a reduction in cost can beachieved. Further, if the driver circuit is provided outside thesubstrate 700, wirings would need to be extended and the number ofconnections of wirings would be increased, but by providing the drivercircuit over the substrate 700, the number of connections of the wiringscan be reduced. Consequently, an improvement in reliability or yield canbe achieved.

[Liquid Crystal Panel]

FIG. 15B illustrates an example of a circuit configuration of a pixel ina liquid crystal panel that is one mode of a display panel. Here, acircuit of a pixel that can be used in a VA liquid crystal display panelis illustrated.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrode layers. The pixel electrodelayers are connected to different transistors, and the transistors canbe driven with different gate signals. Accordingly, signals applied toindividual pixel electrode layers in a multi-domain pixel can becontrolled independently.

A gate wiring 712 of a transistor 716 and a gate wiring 713 of atransistor 717 are separated so that different gate signals can besupplied thereto. In contrast, a source or drain electrode layer 714functioning as a data line is shared by the transistors 716 and 717. Anyof the transistors described in Embodiment 3 can be used as appropriateas each of the transistors 716 and 717. Thus, the liquid crystal displaypanel can have high reliability.

The shapes of a first pixel electrode layer electrically connected tothe transistor 716 and a second pixel electrode layer electricallyconnected to the transistor 717 are described. The first pixel electrodelayer and the second pixel electrode layer are separated by a slit. Thefirst pixel electrode layer has a V shape and the second pixel electrodelayer is provided so as to surround the first pixel electrode.

A gate electrode of the transistor 716 is connected to the gate wiring712, and a gate electrode of the transistor 717 is connected to the gatewiring 713. When different gate signals are supplied to the gate wiring712 and the gate wiring 713, operation timings of the transistor 716 andthe transistor 717 can be varied. As a result, alignment of liquidcrystals can be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 710,a gate insulating layer functioning as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode layer orthe second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode layer, a counter electrode layer, anda liquid crystal layer therebetween. The second liquid crystal element719 includes the second pixel electrode layer, a counter electrodelayer, and a liquid crystal layer therebetween.

Note that a pixel circuit of one embodiment of the present invention isnot limited to that shown in FIG. 15B. For example, a switch, aresistor, a capacitor, a transistor, a sensor, a logic circuit, or thelike may be added to the pixel circuit shown in FIG. 15B.

[Organic EL Panel]

FIG. 15C illustrates an example of a circuit configuration of a pixel inan organic EL panel that is another mode of the display panel.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodesincluded in the organic EL element and holes are injected from the otherof the pair of electrodes, into a layer containing a light-emittingorganic compound; thus, current flows. The electrons and holes arerecombined, and thus, the light-emitting organic compound is excited.The light-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

FIG. 15C illustrates an example of a pixel circuit applicable to anorganic EL panel. Here, one pixel includes two n-channel transistors.Note that the oxide semiconductor film of one embodiment of the presentinvention can be used for a channel formation region of an n-channeltransistor. Further, digital time grayscale driving can be employed forthe pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving will be described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodelayer of the switching transistor 721 is connected to a scan line 726, afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching transistor 721 is connected to a signal line725, and a second electrode (the other of the source electrode layer andthe drain electrode layer) of the switching transistor 721 is connectedto a gate electrode layer of the driver transistor 722. The gateelectrode layer of the driver transistor 722 is connected to a powersupply line 727 through the capacitor 723, a first electrode of thedriver transistor 722 is connected to the power supply line 727, and asecond electrode of the driver transistor 722 is connected to a firstelectrode (a pixel electrode) of the light-emitting element 724. Asecond electrode of the light-emitting element 724 corresponds to acommon electrode 728. The common electrode 728 is electrically connectedto a common potential line provided over the same substrate.

As each of the switching transistor 721 and the driver transistor 722,any of the transistors described in Embodiment 3 can be used asappropriate. In this manner, a highly reliable organic EL panel can beprovided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727. For example, thelow power supply potential can be GND, 0 V, or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 724, and the difference between the potentials is applied to thelight-emitting element 724, whereby current is supplied to thelight-emitting element 724, leading to light emission. The forwardvoltage of the light-emitting element 724 refers to a voltage at which adesired luminance is obtained, and includes at least forward thresholdvoltage.

Note that gate capacitance of the driver transistor 722 can be used as asubstitute for the capacitor 723, so that the capacitor 723 can beomitted. The gate capacitance of the driver transistor 722 may be formedbetween the channel formation region and the gate electrode layer.

Next, a signal input to the driver transistor 722 is described. In thecase of a voltage-input voltage driving method, a video signal forsufficiently turning on or off the driver transistor 722 is input to thedriver transistor 722. In order for the driver transistor 722 to operatein a linear region, voltage higher than the voltage of the power supplyline 727 is applied to the gate electrode of the driver transistor 722.Note that voltage higher than or equal to voltage which is the sum ofpower supply line voltage and the threshold voltage V_(th) of the drivertransistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage higherthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 724 and the threshold voltage V_(th) of thedriver transistor 722 is applied to the gate electrode layer of thedriver transistor 722. A video signal by which the driver transistor 722is operated in a saturation region is input, so that current is suppliedto the light-emitting element 724. In order for the driver transistor722 to operate in a saturation region, the potential of the power supplyline 727 is set higher than the gate potential of the driver transistor722. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 724 in accordance with the videosignal and perform analog grayscale driving.

Note that a configuration of a pixel circuit is not limited to thatshown in FIG. 15C. For example, a switch, a resistor, a capacitor, asensor, a transistor, a logic circuit, or the like may be added to thepixel circuit shown in FIG. 15C.

In the case where any of the transistors described in Embodiment 3 isused for the circuit shown in FIGS. 15A to 15C, the source electrode(the first electrode) is electrically connected to the low potentialside and the drain electrode (the second electrode) is electricallyconnected to the high potential side. Furthermore, the potential of thefirst gate electrode may be controlled by a control circuit or the likeand the potential described above as an example, e.g., a potential lowerthan the potential applied to the source electrode, may be input to thesecond gate electrode through a wiring that is not illustrated.

<Electronic Device>

The semiconductor device of one embodiment of the present invention canbe used for display devices, personal computers, or image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images). Otherexamples of electronic devices that can be equipped with thesemiconductor device of one embodiment of the present invention aremobile phones, game machines including portable game consoles, portabledata appliances, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), and vending machines. FIGS.16A to 16F illustrate specific examples of these electronic devices.

FIG. 16A illustrates a portable game console including a housing 901, ahousing 902, a display portion 903, a display portion 904, a microphone905, a speaker 906, an operation key 907, a stylus 908, and the like.Although the portable game machine in FIG. 16A has the two displayportions 903 and 904, the number of display portions included in aportable game machine is not limited to this.

FIG. 16B illustrates a portable data terminal including a first housing911, a second housing 912, a first display portion 913, a second displayportion 914, a joint 915, an operation key 916, and the like. The firstdisplay portion 913 is provided in the first housing 911, and the seconddisplay portion 914 is provided in the second housing 912. The firsthousing 911 and the second housing 912 are connected to each other withthe joint 915, and the angle between the first housing 911 and thesecond housing 912 can be changed with the joint 915. An image on thefirst display portion 913 may be switched depending on the angle betweenthe first housing 911 and the second housing 912 at the joint 915. Adisplay device with a position input function may be used as at leastone of the first display portion 913 and the second display portion 914.Note that the position input function can be added by providing a touchpanel in a display device. Alternatively, the position input functioncan be added by provision of a photoelectric conversion element called aphotosensor in a pixel portion of a display device.

FIG. 16C illustrates a laptop personal computer, which includes ahousing 921, a display portion 922, a keyboard 923, a pointing device924, and the like.

FIG. 16D illustrates the electric refrigerator-freezer including ahousing 931, a door for a refrigerator 932, a door for a freezer 933,and the like.

FIG. 16E illustrates a video camera, which includes a first housing 941,a second housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andthe angle between the first housing 941 and the second housing 942 canbe changed with the joint 946. Images displayed on the display portion943 may be switched in accordance with the angle at the joint 946between the first housing 941 and the second housing 942.

FIG. 16F illustrates an ordinary vehicle including a car body 951,wheels 952, a dashboard 953, lights 954, and the like.

The structures, the methods, and the like described in this embodimentcan be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

This application is based on Japanese Patent Application serial no.2013-190216 filed with Japan Patent Office on Sep. 13, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising the steps of: forming a first oxide semiconductor layercontaining a single metal element as a constituent element over asubstrate by a thermal chemical vapor deposition method; forming asecond oxide semiconductor layer containing two or more metal elementsas constituent elements by the thermal chemical vapor deposition methodand by epitaxial growth using the first oxide semiconductor layer as aseed crystal successively after the first oxide semiconductor layer isformed; and forming an insulating layer over the second oxidesemiconductor layer, wherein the second oxide semiconductor layercomprises indium, an element M, and zinc, M being Al, Ti, Ga, Y, Zr, La,Ce, Nd, or Hf, and wherein the second oxide semiconductor layer has aconcentration gradient such that, near an interface with the insulatinglayer, the concentration of element M is lower as the second oxidesemiconductor layer is farther from the insulating layer, and near aninterface with the first oxide semiconductor layer, the concentration ofelement M is lower as the second oxide semiconductor layer is fartherfrom the first oxide semiconductor layer.
 2. The method of manufacturinga semiconductor device according to claim 1, wherein the first oxidesemiconductor layer and the second oxide semiconductor layer are formedin a same deposition chamber.
 3. The method of manufacturing asemiconductor device according to claim 1, wherein the second oxidesemiconductor layer comprises a channel, and wherein a gate electrode isformed to overlap side surfaces of the first oxide semiconductor layerand side surfaces of the second oxide semiconductor layer in a channelwidth direction.
 4. The method of manufacturing a semiconductor deviceaccording to claim 1, wherein the thermal chemical vapor depositionmethod is a metal organic chemical vapor deposition method.
 5. A methodof manufacturing a semiconductor device, comprising the steps of:forming a first oxide semiconductor layer containing a single metalelement as a constituent element over a substrate by a thermal chemicalvapor deposition method; forming a second oxide semiconductor layercontaining two or more metal elements as constituent elements by thethermal chemical vapor deposition method and by epitaxial growth usingthe first oxide semiconductor layer as a seed crystal successively afterthe first oxide semiconductor layer is formed; forming a firstinsulating layer over the second oxide semiconductor layer, forming asource electrode and a drain electrode connected to the first oxidesemiconductor layer and the second oxide semiconductor layer; forming asecond insulating layer adjacent to one of the first oxide semiconductorlayer and the second oxide semiconductor layer; and forming a gateelectrode adjacent to the second insulating layer, wherein the secondoxide semiconductor layer comprises indium, an element M, and zinc, Mbeing Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf, and wherein the second oxidesemiconductor layer has a concentration gradient such that, near aninterface with the first insulating layer, the concentration of elementM is lower as the second oxide semiconductor layer is farther from thefirst insulating layer, and near an interface with the first oxidesemiconductor layer, the concentration of element M is lower as thesecond oxide semiconductor layer is farther from the first oxidesemiconductor layer.
 6. The method of manufacturing a semiconductordevice according to claim 5, wherein the gate electrode overlaps thesecond oxide semiconductor layer with the second insulating layerprovided therebetween.
 7. The method of manufacturing a semiconductordevice according to claim 5, wherein the first oxide semiconductor layeroverlaps the gate electrode with the second insulating layer providedtherebetween.
 8. The method of manufacturing a semiconductor deviceaccording to claim 5, wherein the thermal chemical vapor depositionmethod is a metal organic chemical vapor deposition method.
 9. Themethod of manufacturing a semiconductor device according to claim 5,wherein a thickness of the first oxide semiconductor layer is0.1-atomic-layer thick to 20-atomic-layer thick.
 10. The method ofmanufacturing a semiconductor device according to claim 5, wherein thefirst oxide semiconductor layer contains zinc.
 11. The method ofmanufacturing a semiconductor device according to claim 5, wherein thefirst oxide semiconductor layer and the second oxide semiconductor layerare formed at a temperature of 400 ° C. to 800 ° C.
 12. The method ofmanufacturing a semiconductor device according to claim 5, wherein thesecond oxide semiconductor layer comprises a channel, and wherein thegate electrode overlaps side surfaces of the first oxide semiconductorlayer and side surfaces of the second oxide semiconductor layer in achannel width direction.
 13. A method of manufacturing a semiconductordevice, comprising the steps of: forming a first oxide semiconductorlayer containing a single metal element as a constituent element over asubstrate by a thermal chemical vapor deposition method; forming asecond oxide semiconductor layer containing two or more metal elementsas constituent elements by the thermal chemical vapor deposition methodand by epitaxial growth using the first oxide semiconductor layer as aseed crystal successively after the first oxide semiconductor layer isformed; forming a first insulating layer over the second oxidesemiconductor layer; forming a source electrode and a drain electrodeconnected to the first oxide semiconductor layer and the second oxidesemiconductor layer; forming a third oxide semiconductor layer by thethermal chemical vapor deposition method and by epitaxial growth usingthe second oxide semiconductor layer as a seed crystal, the third oxidesemiconductor layer overlapping side surfaces of the first oxidesemiconductor layer and side surfaces of the second oxide semiconductorlayer in a channel width direction; forming a second insulating layerover the third oxide semiconductor layer; and forming a gate electrodeover the second insulating layer, wherein the second oxide semiconductorlayer comprises indium, an element M, and zinc, M being Al, Ti, Ga, Y,Zr, La, Ce, Nd, or Hf, and wherein the second oxide semiconductor layerhas a concentration gradient such that, near an interface with the firstinsulating layer, the concentration of element M is lower as the secondoxide semiconductor layer is farther from the first insulating layer,and near an interface with the first oxide semiconductor layer, theconcentration of element M is lower as the second oxide semiconductorlayer is farther from the first oxide semiconductor layer.
 14. Themethod of manufacturing a semiconductor device according to claim 13,wherein the thermal chemical vapor deposition method is a metal organicchemical vapor deposition method.
 15. The method of manufacturing asemiconductor device according to claim 13, wherein a thickness of thefirst oxide semiconductor layer is 0.1 -atomic-layer thick to 20-atomic-layer thick.
 16. The method of manufacturing a semiconductordevice according to claim 13, wherein the first oxide semiconductorlayer contains zinc, and wherein the third oxide semiconductor layercontains indium and zinc.
 17. The method of manufacturing asemiconductor device according to claim 13, wherein the first oxidesemiconductor layer, the second oxide semiconductor layer, and the thirdoxide semiconductor layer are formed at a temperature of 400 ° C. to 800° C.
 18. The method of manufacturing a semiconductor device according toclaim 13, wherein the second oxide semiconductor layer comprises achannel, and wherein the gate electrode overlaps the third oxidesemiconductor layer in a channel width direction.
 19. The method ofmanufacturing a semiconductor device according to claim 1, wherein thefirst oxide semiconductor layer is formed using an organometallic gascontaining zinc and a gas containing oxygen, and wherein the secondoxide semiconductor layer is formed using an organometallic gascontaining indium, an organometallic gas containing gallium, theorganometallic gas containing zinc, and the gas containing oxygen. 20.The method of manufacturing a semiconductor device according to claim 1,wherein the second oxide semiconductor layer is formed while a flow rateof a source gas is changed, whereby a composition of the second oxidesemiconductor layer changes in a thickness direction.
 21. The method ofmanufacturing a semiconductor device according to claim 1, wherein anelectron affinity of the second oxide semiconductor layer is higher thanan electron affinity of the first oxide semiconductor layer.
 22. Themethod of manufacturing a semiconductor device according to claim 5,wherein the first oxide semiconductor layer is formed using anorganometallic gas containing zinc and a gas containing oxygen, andwherein the second oxide semiconductor layer is formed using anorganometallic gas containing indium, an organometallic gas containinggallium, the organometallic gas containing zinc, and the gas containingoxygen.
 23. The method of manufacturing a semiconductor device accordingto claim 5, wherein the second oxide semiconductor layer is formed whilea flow rate of a source gas is changed, whereby a composition of thesecond oxide semiconductor layer changes in a thickness direction. 24.The method of manufacturing a semiconductor device according to claim 5,wherein an electron affinity of the second oxide semiconductor layer ishigher than an electron affinity of the first oxide semiconductor layer.